I think the focus on BJJ here sort of muddies the discussion.
I think Submission grappling, what a lot of people would consider "BJJ", is no longer a viable option as the main offensive tool for a top-tier fighter, but is still a necessary secondary offensive tool, and obviously a core defensive tool.
Positional grappling - takedowns, pressure, control, and scrambles, or what a lot would think of as "wrestling" is THE most important core skillset both offensively and defensively for a complete MMArtist, unless they're Anderson Silva.
Thing is, as Soojooko pointed out, BJJ isn't just submission grappling, it involves a lot of positional stuff as well (mind you, I doubt Maia's takedowns got that good only thanks to dropping down. I'm pretty sure he's been training with wrestlers.) And BJJ isn't the only martial art that teaches submissions.