Not everyone needs a microwave mountaintop site or even a decently tall tower.  bear in mind the items that would likely be hosted in sites such as these would be more of a community resource such as many voice repeaters are (and could be collocated with a club that had decent resources)  The resources and hardware at such sites would be for the most part pieces of the network core, not the outlying networks that connected to it and used it's backbone..
 

some will be somewhat limited, this is true but when it comes to effective radiated power and RX gain I can easily outdo your 25W of 70cm at the antenna feedpoint with a watt at 5760 or 10250 measured at the feedpoint of an antenna of similar physical size.  just because some may not live on top of a 10,000 ft mountain peak with multiple 1000ft self supporting tower structures does not mean they can't make good use of amateur microwave frequencies.  such antennas get smaller and easier to put up higher as the frequency goes up.  all that's needed is that you can link to the next node from you or into the backbone network.  some may be so limited as to only be able to do so by internet tunneling.  but that doesn't mean that those who are not so limited should not build the network.  Amateur microwave at 2.4GHZ and above is the proper place to do so.  It gives the performance we need with hardware most will find affordable and easy to use and implement.  If you're living on a city lot, my guess is that your next hop neighbor ought to be much closer than if you're living 40miles out of town.  also in more densely populated areas you will need greater network capacity and bandwidth to properly serve each user an equal experience.  20 people contending for use of a 56k link is much different than 1 user having it to themselves.
 

on a per user basis, yes 56k may serve as a minimum bar but try sharing that simultaneously with 20 other users.  get 20 monkeys together in an attempt to recreate the works of shakespere and you'll have that line saturated for an infinite amount of time (ref rfc2795) as even they can likely be trained to type faster than that.  I know I can certainly read what one of them could type at that speed.  That said why shoot for a minimum even if it is 5-6x faster than what we have stagnated upon for 20+ yrs.  People didin't jump at 56k then, why should they now?
 

yes, Dstar has effectively priced itself out of the market as to costly, to little, to late while being an orphan that no one except amateur radio uses.  compare that however to the cost of a 1M dish and a professional grade 802.11a 30dbm radio that goes from ethernet to RF.  the 1M dish can be had for 89usd and the radio for about the same.... so for under 200usd you have a setup that can link that even in a somewhat rural setting ought get you to your nearest ham neighbor.  Compare this, faster and cheaper to the UDR56k.  I think people would go for the faster, easy to set up 200USD solution.
 

Why is actually easier to answer than it might seem.  We'll start with 47CFR97.1 with which you and everyone on this list and with access to AMPRNET ought be familiar:

§ 97.1   Basis and purpose.

The rules and regulations in this part are designed to provide an amateur radio service having a fundamental purpose as expressed in the following principles:

(a) Recognition and enhancement of the value of the amateur service to the public as a voluntary noncommercial communication service, particularly with respect to providing emergency communications.

Building such infrastructure enhances our ability to serve the public especially in providing emergency communications in time of need.  one of the key uses for such infrastructure is tha ability to show up somewhere and light up communication facilities between disaster sites that are capable of moving voice and computerized data rapidly and efficently which is also usable by those with little or no tech training.  i.e. they attach their laptop to the network at the disaster site and are online with all other disaster sites they need contact with and have simultanious voice and data connectivity between sites.  By building such a network we augment our ability to justify our existance.

(b) Continuation and extension of the amateur's proven ability to contribute to the advancement of the radio art.

by building such a network we work to advance the radio art thus satisfying another point of why we are allowed to exist.

(c) Encouragement and improvement of the amateur service through rules which provide for advancing skills in both the communication and technical phases of the art.

by building and operating such a network we advance our skills in communication and technical phases of the art such as RF linking and TCP/IP network operation and optimization.  We serve as a training ground and build our own skills along with the skills of others in the area of data communications.

(d) Expansion of the existing reservoir within the amateur radio service of trained operators, technicians, and electronics experts.

by building such network we expand the existing reservoir within the amateur radio service of trained operators, technicians, and electronics experts not only training those in technical arts they can take elsewhere but also bringing greater numbers into the hobby for which this is their primary interest in communications (not everyone is into 30second weak signal contacts or chasing DX on 20M).

(e) Continuation and extension of the amateur's unique ability to enhance international goodwill.

 By building such a network on an international basis we create yet another means to extend the amateur's unique ability to enhance international goodwill.
 
those right there ought to be reason enough to get building... but I'll add a few more:

Preservation of the precious frequency allocations we have available for our use - anyone remember when we once had 220-222Mhz available to our use and how due to perceived non-use it was reassigned for land mobile use resulting in a 40% loss of the spectrum in the 1.25M band?  We have microwave bands that go largely unused and other services that would love to run us out of them.  By building such a network we put these bands to effective good use

again, It serves as an avenue to bring more people into the amateur radio fraternity.

it allows us a platform upon which creative applications can be built.  imaging having a minimum of 24 voice channels between each and every mountaintop or backbone node as well as reasonably high data bandwidth.  that kind of trunking could be used for repeater linking or even allowing multiple conversations over a wide area to be carried on simultaniously.  once voice is bits, it's just data like anything else on the network.  voice is only a start, the rest of the applications that may be built are limited only by the imagination of the amateur radio AMPRNET community and their willingness to advance the technical arts.

Eric
AF6EP