There are a lot of people who are going to own nest devices that they had professionally installed, as many of these purchases push for using an on-site technician service to install for you (try buying any of the smart thermostats on Amazon, they all have an option for professional installation).
It can be hard especially if you don't have a replacement just lying around.
I'm sure I'm missing something, but in the past when I've replaced thermostats it's pretty easy. Easy to find a replacement and easy to do the work. I know that everyone has different levels of skill here but this is less involved than, say, replacing an outlet.
A replacement compatible with the heating that may have been installed 10 years ago by a company that went out of business and from a manufacturer that also went out of business.