I think there's two things going on really.
First, from the OP's reference to feeling the need to switch armies in order to stay viable because of faction locked units, that sounds like FOMO. Really, almost every army has at least one proven viable tournament winning archetype. It would seem better to change ones list to that, instead of trying to homogenize armies with unique and play style defining unit options.
Second, I partly agree that there is archetype overlap between a few chapters, such as the Raven wing and White Scars bikers, or Blood Angels and Raven Guard use of Jump Pack infantry. But I disagree that that shared affinity for particular unit types mean they SHOULD have the same unit options. Personally, I think the Company Veterans squad options of jump packs and bikes should be taken out of Legends, just like the SM Captain on Bike was for 9th, and given to DA/WS and BA/RG respectively. I dont think DA having raven wing or BA having DC should disqualify them from having their respective company vets, because each is play style appropriate AND does not equate the Faction Locked unit, one for one. Company Veterans on Bike are not better or worse than Raven Wing Black knights, they are different and fill different roles. Just like CV w.JP's are not better or worse than DC or SG, they just do a different job in the battle plan .