I can't personally speak on the longevity of Sportsters as my 91 only had about 25k miles on it when I sold it, and my current 06 only has 7k, but if you have any questions on Sportster durability, go check out the XL Forum. There are many discussions about accumulated miles on Sportys, here is an example:
Miles on Modern Evo Before Engine Rebuild? - The Sportster and Buell Motorcycle Forum
Many guys have over 100k miles and still accumulating. It may be old technology, but it seems to work. Sure, the power to weight ratio may not be the best, but then how many of us are drag racing our bikes every day?
Maintenance is minimal on the Sportster Evo engines compared to "modern technology", like valve adjustments. What is it every 10k miles for the Vrod before checking and shimming them? A Sportster does not require valve adjustments. NO water pumps, radiators, or cooling fans.
Each model has it's advantages and disadvantages. I don't see a problem with either, and I believe one will get reliability and longevity whether it be from a Street with modern technology or a Sportster with proven archaic simplicity. To each, their own.