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Yep, you are certainly right to a degree, however I know the on one parkwood has been out a few years, and the 27+ version is a new addition to their range. 29ers with big clearances can squeeze the full 3.0 tyres, but generally better to get one designed for the job.
Then your problem is that a lot of new 27+ frames will be using the new 148 boost spacing and therefore awkward for swapping wheelsets.
It's easy to design a frame to take both, but that's very different from trying to stuff fat tyres into something that was never designed for them