Out of Life support was a huge issue in the case I outlined above. I kept things ticking over on my side, but there were no more updates from the original vendor except for bespoke work we had to pay them for to complete annual updates for federal compliance issues. Actually we pooled together with a few organizations in the same boat to do that until we rebooted the failed project.

To give the legacy vendor credit though, It was a legacy product 5 years past its "final" EOL and kept honoring the maintenance agreement and providing ToS updates for a long time. In terms of the database itself, it probably helped that it wasn't their own: It was native to OpenVMS and hadn't substantially changed in at least a decade. Ultimately that made data migration a bit easier since industry tools for migrating from VMS systems had reached maximum maturity by the time we got around to it.

I still have a soft spot for that old system though: It lacked any sort of modern functionality newer than about 1995 and the underlying application has it's nroots in the 60's. But it was fast & I had low level access to do some complex things much more easily than the upgraded system (installed about 6 years ago). You won't get much faster than a well-tuned decades-old system running on modern hardware, at least not unless you need something that can handle medium-to-big data.