I agree with the idea of adding spells that make the beast better.Yes.
If that doesn't work for someone, literally any race with a wolf is the classic ranger pet.
They literally can just provide alternate class features for a subclass just as easily as for a class. This argument is nonsensical and objectively false.
either Mearls is wasting time entirely, or alternate class features are a possibility.
Further, they can literally accomplish the same benefits of this proposal by adding spells to the class spell list that boost a pet. Word it so that they will also work for familiars, and maybe summoned pets, but not for NPC companions, and go.
But giving the pet hit dice instead of xtimes ranger level HP is a simple change, as is making it have a turn and no multi-attack, and that is all that is needed to bring the subclass in line with other ranger subclasses, and can allow the same effective benefit of the proposed build-a-beast if combined with spells that boost the stats of a controlled beast.
Even allowing your reaction to apply to your pet - absorb elements for instance - is there a problem that makes shared spells a 15th level trait? It's the value of the spell that determin's its potency.