Doing quests and leveling up in the game is nice up to a certain point. But as soon as you think about instances and post-Ch4 regions, you will hit limitations and outright paywalls. The game's upgrading system was entirely built on pay to win and it has always been like that. You really don't need skill and experience to beat the strongest bosses here. You need enough diamonds to upgrade your equipment enough for crazy high stats. The game is long past its golden days which were ten years ago. By the time Ch3 was released, the playercount was already decreasing. RoM still generates profit for GF and the upkeep appears to be quite cheap for them, otherwise they wouldn't keep the game running.
The only "p2w" required to do even the hardest instances would be for 3 and 4 runeslots on equipment, and +12 to +16. If I am not wrong most runes in itemshop can also be crafted. That imo is fairly reasonable for an f2p game, with dia promo's and discounts.
There are support/healer classes also which require 0 gear but are still needed in hard mode inis for those who wish to play max lvl hard modes f2p. You can certainly run normal modes with f2p gear with dps classes etc.
Just because they made it possible doesn't mean the instances are designed around the obscene end of the p2w. They don't require the whole party to have +30 t11 gear, and t15+30 wep. If you think so, you need to find better parties, or compensate the lack in gear with skill. On the other hand, if they make ini's that require that much gear, then i'd agree you have a point. But as of now there aren't any imo (that are atleast doable, unlike pantheon hm, where even p2w peak 10 max geared can't hit anything atm).
And if you don't want to buy dias with real money, you can always farm gold using various f2p methods and buy dias with them from those selling, so you can have pretty high p2w level gears while being fully f2p, as quite a few do. p2w does make the process much faster sure.