Given what reception? From whom? The forum? Reddit? They only represent themselves and not the player base as a whole. Of course you should take into account people who pick up content at a later date. I mean it happens for Savage and Eureka for example. Why not BLU?
Yes, it depends on the content. Not all content are meant to appeal to everyone. I do take an issue on how you're creating false categories to arbitrarily split BLU and Savage. You can just as easily call Savage (and any content for that matter) as "something to do" when during a content drought. I remember hearing that criticism towards Eureka too. Huh, it's funny that BLU and Eureka can be called as disposable content, but not Savage or Ultimate. I'm guessing something to do with 'niches', right?
What do you mean that Savage isn't aimed at the general playerbase? They have tried for a while to get more people to do it, and not just the hardcore few. With Ultimate out, they have been trying to make it even more accessible for the casual player. I can't tell you if that's working, since I don't have the numbers. But at least I'm not jumping the gun and calling the attempt a failure and encouraging SE to abandon ship and focus on more popular things that I like.
The fact is that despite fulfilling a niche, Savage and Ultimate are still done by a select few, despite SE trying to make it appealing to a wider audience, yet you're not calling that a failure and waste of time and development. You're even rationalizing it by mentioning how relatively cheap it is to develop Savage/Ultimate, while not giving BLU the same benefit (BLU learns skills that are already in-game, borrows gear from casters, no weapon stats, no need for balance, etc). Why do you suppose that is?