Then there’s the issue of the historical record. Sure everything that was there seems to be there, but you should’ve created a middle layer to retain old links.
These are open source projects that likely have had thousands of contributions made to them over the years. I dare say not many companies are writing their own web software from scratch these days. The World Wide Web was still referred to as such, back when that was feasible.
More to the point, there is a layer that attempts to convert old links. The caveat is that given the variation in which links may have been copied initially, you may have to edit the link for it to work. The most common thing I’ve had to do is look for the t=<NUMBER> sequence in the link, and just take everything else out. That will usually get me to the right place, and I can update the bookmark or whatever once I’m there. For example, this is one I found that doesn’t work:
https://www.literatureandlatte.com/viewtopic.php?f=4&t=53084&p=275525#p275525
We can fix it with:
https://forum.literatureandlatte.com/viewtopic.php?t=53084
The ‘t’ stands for topic and the number is the topic ID, so if that can’t be found then it is likely the link would have been broken all along anyway—likely the topic was merged with another or deleted.
I know change serves a purpose, but in this case everything is different. It would be like rewriting scrivener again and just expecting people to be okay with the changes.
The previous forum software provider was no longer keeping up with the times, so we moved to a new system, when the dependencies it had were so ancient that it was enough of a security risk that our host was phasing them out. So that is your acknowledged purpose—in this case our choice was between sticking with a stagnating system that has been patched to work on newer platforms, or take the opportunity to go with a platform that still has passion in the project and is clearly moving forward.
That said, I question your reasoning. By your logic we should still be running Apple ][ and MS-DOS computers (and of course, whatever you started out with is what a Real Computer is). After all, Microsoft Windows and Macintosh System were a complete rewrite (well sorta, in one case) of the operating system, again, which just expected people to be okay with the changes. 
There are periods of time where the colour of things change, and minor feature clusters are added or modified within a model, and then there are times of change when the whole premise must be rethought—because it’s been 15 or 20 years, and computers went from beige boxes belching hot air out the back and making cat scream noises over a phone line, to things you slip into a pocket, that pull the internet out of the atmosphere at a rate that would take a thousand screaming modems.
This is the way.