It's certainly possible to make a strong case that it is, at least in games where items are important and can be exchanged. There are people whose opinions I respect who are against Real Money Trade. I believe I understand their reasons pretty well, but it seems clear that most of the online gaming world thinks otherwise. For what it's worth, I tend to agree.
Just to be clear, RMT and free to play are not synonymous, although they do overlap to a substantial degree. By my definition, the former is simply the ability to buy in-game items and/or characters using real money, whether directly or indirectly via the step of purchasing a virtual currency. Personally, I don't care if it's possible to cash out or not, although this obviously matters a lot to anyone looking to make a profit.
Basically, it seems like a matter of free enterprise founded on exchanging time for money and vice versa. When you play a subscription game, you pay a flat monthly fee, then progress your character based on the number of hours you invest. It's not a completely linear function. To use myself as an example, I tend to explore and try new things quite a bit rather than grinding for experience and/or farming items all the time. As a result, there are periods, sometimes lasting quite a while, when my advancement is slow. If I happen to decide I want to pick up the pace by buying a better weapon, some bonus XP potions or whatever, I really don't see that as constituting a problem for anyone else.
Yes, I've been in the situation where I grouped with people whose knowledge of how to play was clearly not on par with what I'd expect from seeing their respective character levels or gear. So what? In the very worst cases, I died because they didn't understand or fulfill their roles. But occasional deaths are unavoidable even in solid parties due to my preferred play style; I don't think it's nearly as fun to restrict myself to places and quests where there's never much chance of dieing. Nowadays, with penalties being far less severe than they were in year past, what have I really lost?
Plus that hasn't happened to me very often anyway. People who buy items still have to play (and learn to play), and they far outnumber those who purchase characters. In addition, quite a few titles now allow players to sell cash shop articles for game currency or to trade them for dropped or crafted ones because doing so lets the operator indirectly monetize non-paying users. So having such gear isn't even a particularly good indicator of someone's knowledge or ability.
It's also the case that precluding in-game RMT will reduce but probably won't completely prevent it. There are plenty of third-party sites where people can go. Unfortunately, all of them have potential risks, the most obvious being non-delivery of something you've paid for - and they offer little or no recourse. There's no question I'd rather have the greater sense of confidence and margin of safety that come with the publisher handling the transaction, even if it means the cost is somewhat higher. Indeed, that may not necessarily be so since the unregulated sites are obviously in the business to make money.
To get back to my original question about inevitability, I won't go as far as to predict every game will permit RMT, at least not within a reasonably foreseeable time frame, say the next few years. However, it's hard to see the growth trend changing.
What I do suspect will happen is that even titles that don't allow it will implement additional revenue streams; i.e. they'll offer you more ways to spend your money. We've seen some already. Pre-made characters for sale and premium servers are two examples. Neither is new, but we've yet to see many other ideas, which means we've only scratched the surface.
As to why I expect this to happen, the primary reason is very simple. Only charging a monthly subscription fee leaves a lot of money on the table that could be in the publishers' and operators' coffers. They're in business to make money too, and what's more, those who are aware of North American business history know that most consumer grumbling isn't backed up with subsequent action.
Think of it like this. When character classed are nerfed, how much moaning do you see versus players actually quitting? In a similar vein, if a game people enjoy and have invested hundreds or even thousands of hours in adds a new, optional revenue stream, how badly do they have to hate it before they'll leave? More RMT is inevitable because it supports a lot of possibilities.