Personally, I always feel as though, even though changes must be made (otherwise, why bother remaking the game? Just stick with the original!), a key element of remakes of any kind is that they must retain the features of the originals. I don't want to have to go back and play the originals now that their gameplay is outdated; if I were to go back, it would be so that I may get a nostalgic glimpse at how the original games were. Not to play aspects that were cut from the remake.
So naturally, I do not agree with Masuda at all in his reasoning. At least the Battle Frontier added mechanics that were unique to its facilities; the Battle Maison is just the same Single-Double-Triple-Rotation formula we've been doing throughout the entire main story, but with AI that actually cheats. In that case, I would much rather face against an actual human opponent. Why waste my time with the Maison at all? And yet, I do not feel that the "improved online" is a worthwhile "substitute." Those battles are still the standard Single-Double-Triple-Rotation formula, lacking the genuine variety provided by the Frontier's facilities. The rumors going around that the Battle Spot Doubles rules will not be patched to allow for VGC 2016 online practice definitely doesn't help, either.
Not to mention, during gen 4, I did not IV breed or EV train, yet I was still able to get most certificates from the Sinnoh Frontier, so it definitely does not need to be something that is "hardcore-exclusive." In addition, EV training in and of itself is made extremely "casual-friendly" starting in gen 6, where, even if you don't do horde battles, the Super Training feature is built-in to your touch screen, ready to be accessed at any time. I'm sure with a bit of AI tweaking, the Battle Frontier could be made into something that is accessible to both "competitive" and "casual" audiences alike.
And if they truly feel as though they need to cater to the "casual" audience more, why not include more casual-oriented post-game content to fill the void of omitting the Battle Frontier, at the very least? I deeply apologize, but I cannot lie: no matter how I look at it, I think the omission of the Battle Frontier is just a poor choice, flat-out, and I feel as though the post-game and overall "longevity appeal" of the games suffered for it.