Let me preface this by saying A.) competition is an inherently good thing; and B.) if there is a plugin out there that is in desperate need of some competition, it is W3 Total Cache.

That said, I have to echo and add to Andreas Nurbo's sentiments, above: Why should I buy this plugin when there is one out there already that's not only free but in actuality does more...? The stretching of the truth on WP Rocket's features page as regards what WP Rocket does and W3TC does not also smacks a bit of WP Rocket's devs not really knowing how to answer this question. Which means, one must conclude, that there isn't a really good reason to purchase and use it instead of W3 Total Cache.

Again, I, myself, absolutely love the idea of giving W3TC a run for its money. That said, I see no evidence that WP Rocket -- in its current iteration -- is capable of doing that. I see no salient, outstanding selling point.

My suggestions to WP Rocket's developers would be as follows:
1.) First and foremost, your plugin should not only do everything that W3TC does (which it does not: cf. combine-only for .js and CSS, custom file placement, DB Cache, Object Cache, etc etc), but it should do it better.
2.) Have the plugin capable of doing something really and truly innovative: The capacity to inline critical CSS and defer the rest on a page-by-page basis comes to mind. (You would actually be killing 2 birds with 1 stone with this: W3TC AND Autoptimize).

^ Implementing those two suggestions alone would ipso facto make WP Rocket a caching plugin worth the public's money.

Best,
AJ