I didn't mean to make it sound like it's no big deal, but I stand by my statement that the grid matters more:
FLB-ing increases your base stats as well, so if half your atk comes from your weapons, a 10% increase from base stats from FLB-ing all your weapons is a 5% increase in dmg - this will be multiplicative with the additional assault you get (as well as exceed and vigor.) The 205% assault (I actually got 193% but doesn't really matter) becomes around 179% to 190% depending on how you do your calc, which technically is feasible with full FLB. Also, assault isn't everything. Don't ignore things like exceed and vigor that might come as a second skill on FLB, exceed in particular matters a lot in late game when everyone has tiara sets and therefore short burst cycles. You should see a big change in dmg if you try doing dummy with and without exceed weapons (this isn't just about hitting cap either, exceed acts as a separate multiplier such that a single large FLB exceed boosts your burst dmg by 10%-ish if you're not using PF.)
Going from full SSR to full FLB grid should boost your dmg by 20%-ish just from base stats and assault gain, and much more when you include exceed and vigor even if we ignore endurance skills like defender and ascension. Put differently, someone with P2W eido with only a full SSR grid is not gonna be able to compete with a vet with a 45% eido but has a full or near-full FLB grid, even if they're all event FLBs and no phantoms.
Actually, if we're talking about 'meta' grids, you most certainly are NOT guaranteed to end up with a 'meta' grid without paying since most of them involve bricking many dupes of certain hime or even draconic eye weapons. If we're talking about just a full FLB grid, sure, but everyone can also eventually farm the guardian eido, which can be quite powerful with a full FLB grid, meta or not.
Game progression isn't hinged on whether you have a P2W or not though, it just raises your ceiling and how quickly you can clear stuff, but that is true with any SSR you draw - if you draw good ones that work well together, you're suddenly more powerful. For complete f2p, you're never guaranteed any himes either regardless of how long you play (heck, even whales aren't guaranteed all the best himes, the number of mtix is limited after all.)
That's the cost of 2 mtix for something that is arguably more valuable than 2 mtix though. Yes, that is expensive for a lot of people but keeping up with the meta for a lot of games can get pretty expensive, not just KH. If you just want to continually improve your teams though, f2p is still viable.