However bringing it on this early on a project's life is often unnecessarily depressing and out of scope: it is a thing to acknowledge, but it only matters once everything is almost ready and the project truly is coming out and maintenance becomes an issue.
Compatibility in the other hand is something that can't be properly dealt with: the syntax is so different and MyBB so dependant on specific syntax on many places that it is probably impossible to support but a handful of existing plugins that add template changes. So it is obvious to me that unless people really fall in love with the new theme it is unlikely there will be other support than keeping things up with MyBB's upgrades.
Also as it seems my customization background needs to be made aware: I used to have a seamless design and seamless login integration between phpBB2, WordPress and MediaWiki. I had to unconstruct it once phpBB2 hit EOL and WordPress login integration plugin halted updates immediately after that point. Since then I had a few years I didn't have enough time and energy to do as big customizations or even give my biggest site the attention it deserves. Which is something I've started to fix now that I do have the time and energy.
I've learned this kind of focus on "what is important right now" the hard way and I don't want to waste too much of my energy on things that don't matter right now. Thus while all this is an interesting topic on itself I wish we could keep this thread on the topic that it is about.