Yeah that’s different. If the OP actually owns the business, then it’s time to look for new software or build better software.
OP already said that this software was written to run highly specialized equipment that is produced by one supplier (the same supplier that writes the software). There is no "new/better software" that OP can look for.
Being tied to software that cannot scale is a time bomb for the business. If the OP actually cares enough about the business they own it’s time to effect change in the way things are run.
The software actually does scale, but it requires more powerful hardware to do the level of analysis that is now being demanded by OP's clients. The software handles this quite well, based on what OP has said.
The company that manufactures the equipment and writes the software is still in business and still supports the software - but like pretty much all instrumentation software, the software is written for a specific platform and is only fully supported on that platform. The most likely reason why no new versions have come out since 2015 is because the platform on which that software is meant to run is still current and fully supported. This is common in the instrumentation industry, and it is not because instrumentation suppliers are cheap or lazy, it is because this kind of software
needs to remain stable and untouched unless there is an absolute need to change it.
The only thing in this situation that is a "time bomb for the business" is using an unsupported platform to run a critical piece of hardware. OP has gotten away with doing so so far, but it is absolutely not something that should ever be recommended. The one and only proper solution for OP is to work with the software vendor to determine the proper hardware and software requirements he needs to put in place to be fully supported by the manufacturer.