Yeah we are, but alas this will be an answer, not a solution for you.
I'm also using a language from the 90s, but one even more niche than Delhi. We have desktop systems, and Web systems, and while they share a lot of source code, the UI procedures are different.
The Ide is still maintained an updated, but of course falls well behind visual studio now.
So for you to switch to this makes even less sense than staying on Delphi. We're under similar pressures to "rewrite in something more modern" but fortunately (for us) the business doesn't have the pockets that sort of project would consume.
Technically there's little reason to change, Functionally the language let's us program anything we want, so apart from the perceived "long term security" of rebuilding from scratch in c# there's not much upside.
For what it's worth we've built the system mostly with 1 developer, although on occasion as many as 3. The primary developer has changed once, and that was a smooth process so it can be done again if necessary.
I write all this to encourage you. If what you have is the best option, then stick with it, swapping bits out slowly as you need to. Make a transition organic, not one giant project. Code is just code. Customers don't care about the language, they don't care about the code, they only care about the solution to their pain.