This is 100% a marketing strategy to neg and force players to upgrade and pay more money
I don't think it is, but I don't mean that in a nice way.
Rather, Beyond, since 2020, have shown an absolutely rank inability to do certain things in what they regard as a "proper way" (despite continually promising to improve their tech), and have also consistently since about then taken an "if we can't do things the 'proper way', we won't do them at all!".
I would bet money that there is no way in their current DB system to flag a magic item or spell as Legacy, and control its availability on that basis. Now, a normal company, run by sane people, would simply go through and put (Legacy) or (2014) after the names of all the 2014 versions of items/spells, and just make them available on that basis (because they definitely could - and do - link them to ownership of the 2014 PHB/DMG). But that isn't how Beyond have operated since 2020 - they've been completely resolute in refusing to do anything sensible/sane/reasonable/helpful-to-paying-customers about stuff they can't trivially do the "proper way" (a nebulous and largely meaningless concept) using their
current tools. They've moaned and moaned that doing things like this would cost time they could be spending working on newer material, and that one day they'll have a "proper" way to do this, and
maybe then they'll do it. For example, the Supernatural Gifts from the Theros setting, which could easily have been rendered as Feats for people who had that book - esp. given Beyond has an easy-to-use override to arbitrarily add Feats to a character (and has done since early on) - the Beyond team absolutely point-blank refused to do this, whilst simultaneously saying that if anyone else did it via the custom Feat system, and shared those Feats, they'd be deleted/unshared. So the Beyond team's solution to their own combination of technical incompetence and frankly, best case laziness, worst case impossibly bad management/prioritization, was to force literally every DM/group who wanted to have those supernatural gifts to INDIVIDUALLY use their fiddly and annoying system to build each one out, and then carefully not share them outside their group(s). And their excuse was "Well one day we'll be able to do it 'properly', so it'd be wasted effort to do it the wrong way!". I mean, as someone who works with IT in a professional capacity, this is one of the most insane and not just unprofessional but actively anti-professional answers I've ever heard, but they've been consistent about it.
So I strongly suspect the same demented mindset is at work here with the spells/items.
It doesn't actually benefit them financially, note - because they're kind of giving away 2024 content for free (indeed you need to spend effort, not money, to retain the 2014 versions). So that doesn't make sense as an explanation. But if we consider this incredibly perverse attitude they've displayed before, which seems to be cultural to Beyond management, then it's quite familiar.