"Free to play means pay to win." In an inversion of a classic gaming trope, Free-to-Play games often, for a few dollars more, allow newer players to buck their natural disadvantage and pwn, instead of being pwned. This breeds conflict, as veteran gamers with hundreds (thousands, even) of hours dedicated to honing split-second reflexes and lightning-fast heuristic calculations find themselves hopelessly outgunned by some sweaty neckbeard with mommy's credit card. In Hansonian terms, the elite gamers are akin to highly skilled foragers who are very good at hunting under egalitarian terms; the only asymmetries are in skills developed through practice (and I encourage you to watch the finals of a Starcraft tournament so you can judge for yourself how refined are some of these skills). In a FtP game, microtransacting players who buy powerful gear are like farmers who've discovered the cheat code of "let's just build a fence around these goats instead of running around the woods all the dang time."
The foragers in this metaphor resent the farmers for being contemptible cheats (judging by the ample scorn heaped upon "casuals" by the more dedicated gaming community), while the farmers send valuable signals to developers.
Valuable signals? Hear me out. Economics isn't always 100% consistent, and a great many economists disagree on a great many things, but one point of large agreement is something called the "equimarginal principle." That's a jargon-y way of saying that people economize on costs, that they strive to get the biggest bang for their... well, it's misleading to say "buck"... for the value of what they give up, be it time, effort, attention, or claims on other resources. Sometimes it's a buck, but sometimes it's just whatever else they might have been able to do. People perform their "felicific calculus" and pursue the greatest reward at the lowest cost. Since everyone's got a different schedule of opportunity costs, microtransactions offer game designers the chance to let players reveal their margins. For some folks, grinding 100 hours for loot is hunky-dory: perhaps they don't have much else to do with their time. For others, dropping $10 to get the irl cash store version is more sensible.
And the proliferation of FtP games means that the economic profits are sure to be wrung from the system. Competition and all that, don't you know. Of course, this is great news for serious gamers. Diminishing returns for FtP games means that traditional, subscription-only multiplayer titles still have a future. While the FtP market is quickly becoming saturated (we might even be a few years past the threshold on that), serious gamers might well find solace within titles that raise a paywall to keep casuals out. Competition permits, even encourages diversity.
Are FtP games euvoluntary? So long as developers aren't hampered from responding to market signals, I'd say yes. The alternative is to try to cram the genie back into the bottle, and that seldom ends well for anyone. Especially the genie.