[AlaskaErik]

The life expectancy of any weapons system is dependent primarily upon the counter systems of its nation's adversaries. Although it may be in front line service for another decade, in my opinion fifty years for any ground based platform is wildly optimistic.

The B-52 entered service 48 years ago. Those still in service are 40 years old and they are expected to remain in service until 2040, and possibly even beyond that. Granted, it's not ground-based, but no reason why the M1A1 can't be around that long.

[Ronaldus Magnus]

In both cases the underlying issue is in maintaining superiority. The B-52 is only successful in theaters where air superiority can be maintained by fighter aircraft and ground forces (as many of the former residents of the Hanoi Hilton can attest). The tank generally has the primary responsibility for obtaining battle field superiority and is therefore much more vulnerable to obsolescence. If tactics were to change to the point where air based ground attack aircraft were tasked with obtaining and maintaining battlefield superiority, then fielding an antiquated tank might be more acceptable. In that scenario, however, a main battle tank will no longer be required (as the "Stryker Likers" would advocate).