If AMD had launched amd64 server chips those vendors onboard we might have had a different 64-bit x86 market. And those vendors would have had a chance to succeed or fail on their own merits instead of due to dead end chip.
Before Intel's decade of humiliation with their 10nm fiasco and before Zen, AMD had a hell of a time getting ISVs and IHVs on board in the server space with producing them, certifying them, etc.,. Which wasn't entirely irrational, Intel was much more mature in a lot of ways in their hardware and software stacks, they were a big company, perceived as less risky, etc.
So it's not too surprising that those companies didn't jump to Opteron. Some did consider it, mind you. Though AMD went through doldrums of their own after Opteron (with Core2/Nehalem-ish era) where they shat the bed with their cores and were pretty handily beaten by Intel for quite a long while until Zen came along -- so that is possibly some vindication for the risk adverse by not going all in on Opteron.
Of course in the alternative history Intel might have done something else in place of IA64 that competed with amd64...
But as you say, the inertia in servers is huge, so Opteron gained market share very slowly, despite being by far the most performant server CPU that was available.
Where I worked at that time, we had big server farms with Sun or Fujitsu servers, which were disliked by design engineers like myself for their slowness. We had to make great efforts to convince the management to buy a few Opteron-based servers. Of course, after we got them they ran circles around the Sun or Fujitsu servers, a single dual-socket Opteron could be much faster than an 8-socket Sun server whose cost was many times greater.