I haven't got a yellow book yet, but I plan to get one eventually. I'll probably be in Thailand for a couple more years, but it's unlikely to be more than that. I don't know how COVID vaccinations are going to play out in the long term. However, based on my experiences with Thai Gov IT so far, I have zero confidence that Mor Prom will be stable and available indefinitely, or that the QR codes will continue to work indefinitely.
If/when they introduce some new system which half works some of the time, it may easy to switch over if you are in Thailand with a Thai phone number, (or maybe not), but I don't expect they will make it easy for anyone who has left.
The yellow book might not be much use now, but it may end up being the only accessible official record I have of my vaccinations here.