It almost certainly slow things significantly, though I won't assert a threshold for "insanely." :) If their updates are so labor-intensive (or their team is so small) that they can only manage a few feature and amp updates every 6 months, an additional parallel development roadmap is absolutely going to slow it further. Finite resources mean trade offs.
While I am making the case for why I don't consider it to be desirable on modelers (for the manufacturer), I personally don't care if any modeler adds capture, so long as: 1. They prove me wrong and it doesn't slow down feature enhancement, amp/effect model addition, etc., and 2. It doesn't hit my interface at all unless I'm intentionally using it.
The QC has been stating and not delivering (or delivering very, very late) from day one, too. I wouldn't use them as the exemplars of engineering effort planning and execution.
It's plausible, yes. But that's a whole different parallel development roadmap (once you fork, you're owning a new product - but still wanting to maintain some connection to the trunk for reasons I mention below), and it comes with a lot of pitfalls. Will you make sure players would be able to use all the other NAM captures out there? There would have to be extensive additional testing to accomplish that to be sure - on an ongoing basis. And if you abandon the rest of the NAM world, how much have you added? Few people actually make captures, so much of the point is the ability to buy/sell/exchange them.
It's all pretty interesting. Then there's the marketing angle. On the one hand, more is more. On the other hand, you counterfeit yourself a bit, and maybe a lot. If you add captures and people like it better than your modeling in the ultimate A/B test (on the same device), your flagship product just became an extremely expensive Tonex with some extra effects. The branding people probably hate the idea. They're trying to sell the notion that "our brand has secret sauce tied to our skills (and in Fender's case, history) that cannot be matched." To go commodity capture implies that the secret sauce doesn't really matter as much as they say it does.