As jussaskin said, it can happen to them too. For example, VM/370 was among first virtualization schemes that could do all kinds of things PC/server virtualization bragged on later. Many key apps/OS's might be put in it. Yet, you'd be hard-pressed to run one of those with today's IT budgets if you couldn't afford a mainframe. Likewise, quite a lot of PC virtualization vendors came and went. The survivors occasionally have compatibility issues or corporations start going in different directions with less support of the older product. The product itself is often tied to a specific OS or HW combination that might go away.

One can use virtualization to help future-proof apps. IBM's System/38 (later AS/400, iSeries, and IBM i) did this for decades. However, to be fully future-proof (esp for vendor level), the virtualization SW/HW solution itself needs to be immune to the problem. That means legally, easily-cloned HW with same for software and virtualization layer. Closest thing maybe NOVA microhypervisor on Loongson-style OSS CPU with x86 emulation if it's x86 apps. RISC-V w/ virtualization support otherwise.

Guess what future-proof virtualization all the mainstream approaches aren't using, though? ;)

If devs have to download a VM running an old OS with old tooling, the barrier to entry is getting quite high. I wouldn't touch an OSS project that made me do that, I would assume it was effectively dead.

Less so, I agree. But give it time - older VM image and container formats will probably rot as well.

While for current software, which depends to at least some degree on remote services (cloud?) This won't help at all.

Even though the VM image and tooling may still be runnable, the world around it with which it must interact continues to evolve and can leave your software obsolete.