[Ursus Maior]

I think, the bigger picture is important here. SPAAG projects died with the Peace Dividend for obvious reasons. Germany was lucky to have one of the best systems in the world (if not the best, at least in the west) and kept the Gepard running until 2011. Combined with Roland on its Marder IFV chassis, that was a very solid protection level.

In an alternate history where the USSR remains a larger thread than Russia during the 1990s was, even in a T2K v. 4 variant, where a short Peace Dividend seems to have been the case, SPAAG systems would have probably been a thing for most NATO armies. So, the usual question is, what to buy. There is DIY and OTS. All do-it-yourself projects are costly and can fail (cf. the M247 Sergeant York), but gains to national knowledge and economy are a clear pro. OTS solutions were available back in the day with the Gepard an obvious solution and the Marksman system a less known one. Still, the US Armed Forces usually had problems buying these things from abroad, even from their allies, with Roland being a notable exception.

Solutions from within the USA were available though and either failed historically mostly because of the changing political climate (ADATS on M113 or Bradley) or were bought in only small numbers. The M6 linebacker and the M998 HMMWV Avenger as well as the M1097 Heavy HMMWV Avenger respectively were bought, but did not perform as well as other solutions, if I am correctly informed. M163 VADS always was only a stop-gap solution.

Buying Marksman could have been a solution, though. Buying British instead of German might have been politically easier, especially if one would have used older M48 or M60 Patton tanks as a chassis. The former was offered as a solution for Marksman by Marconi Electronic Systems from ca. 1994 onwards, the latter was never offered, but I do not see, why it could not have worked.

While keeping the M48 chassis alive for another 10-20 years might not seem like a good idea, Marksman would have been a cheap solution that could offer first tier results (in the mid-1990s). The M48 chassis is well suited for the job, thousands were available readily, since the M48 was becoming clearly obsolete as an MBT, even with all upgrades available (L7 105 mm gun, MOLF laser range finder, thermal sights, modular applique armor or even ERA and a new MTU power pack). As a SPAAG a new power pack and the Marksman turret would have been more than sufficient, though. In an alternate history, where the USSR remains a real, though diminished threat, it is likely that the Detroit Arsenal would have remained fully operational as a tank factory, allowing for a swift modification process during the 1990s.