FWIW, look at it this way.
It's not like the grind is done in any way that impedes your normal activities in the game. The game is about runnin', shootin', jumpin' and lootin'. So whether you're grinding defense missions for Rubedo, grinding Boss missions for a BP, or just grinding for mods and void keys, yer grinding. Grinding for BPs is ultimately easier because a) the boss drop tables are about 3 to 5 items and b) Materials drop in regular, known quantities. Consider that grinding for a particular rare mod is way, way less predictable. I had to do a serious 4 to 5 hour grind session to get the Ashe BPs (which I'd actually gotten once already but a mislabeled item confused me.) Once you've done it, it's done. The rest pretty much comes in the normal course of play.
My issue with the game still currently remains that everything in it right now, even Void Missions, still feel like the training for something more. The end game hinges on new content, and in particular, new drops and/or progression systems that get introduced. Forma, Polarity and the Guild Hall are all some nice long-term goals to play toward...but if you're a 1 Warframe kinda guy, you start running out of things to be motivated to do. The only true challenge I've run into was doing a Tower III Void Key mission. Those guys were ridiculously hard and your weapons did fuck all for damage. So that was really fun, by way of comparison to say a boss fight, which is scripted to be hard.
But I don't know if the game has much of a future if it can't evolve beyond its current form of corridors, dudes and killing. The pieces are all there for them to iterate on it and make more refined gameplay experiences and scenarios. It's a matter of whether they make the time to do it, or get stuck in a loop of endlessly trying to crank out more of the same content to keep the F2P engaging. Like, an open, non corridor-based level would be pretty awesome to run around in.