What is the goal?
If having an open whitelist were some kind of panacea to convert the masses into competitive gamers then MyM would be overflowing with massive numbers. It's not.
I'm interesting in seeing other types of maps brought back in, AD, even some PL, but that's in MyM too and it makes little difference.
The 6v6 game isn't going to be made appreciably better by allowing an extra unlock or two and it could be made a lot worse if the wrong ones are let through, and the true nature of how an unlock changes the game often doesn't become apparent until a really important match. Remember the Quick Fix ESEA final. What reward is there for taking this risk?
Forget Valve making any decisions based around or supporting established competitive activity. They obviously still do not get it. It's clear now that MyM was a least-effort-required attempt to provide a feature Overwatch was offering, not the sea change we all hoped for in their attitude to competitive.
Why break something people have worked so hard to create and establish? If meaningful Valve support and active item balancing were going to materialise as a result of going no-whitelist I'd be for it, but those things simply won't happen.