the much more valid comparison is obviously highlander, over which prolander is a huge and obvious improvement.
Whether the format is an improvement in gameplay or not isn't too important, it isn't an improvement in the space that Highlander actually provides.
Highlander, since the ETF2L Highlander Challenge, has both at best played an important role in getting people's feet wet in competitive with plenty of people transitioning over to 6s and at worst helped make the general pub community accept competitive formats easier.
Highlander is a format which is:
a) completely free, 100% of the time
b) all 9 classes are forced into usage, there is no meta that makes 1 class unplayable because you have to play it.
Which means Highlander can grab pub players regardless of class and regardless of money. So long as Prolander has no 100% free league structure option and so long as Prolander is not 9v9 1 of each class guaranteed and is thus susceptible to a class meta taking place [read: this one will always exist] and thus precluding people who main certain classes from moving up all the ranks, it can't be a direct improvement over Highlander's space in the community.
This is okay though, because multiple formats can exist, even if the overall competitive population is declining we still have people who are doing 4s, 6s, 9s, Ultiduo tournaments, and now 7s.
But, Highlander has an important position within the community regardless of if it isn't someone's cup of tea. There have been plenty of 6s players - both currently in ESEA and formerly in ESEA, across all divisions from Open to even Invite - who basically got their start in Highlander and would have either not gotten into 6s at all or taken a substantially longer time to get there without it, including utility class players who transition into 6s cookie cutter classes.
Mind, I don't mind Prolander. I think it's fun and Sigafoo is cool, but I think the formats just offer significantly different things and it's a bit short sighted to say that Prolander is a huge and obvious improvement in every facet.
I think in essence this is mostly /u/saamtf 's point.