They are a problem because the in-game economy is deliberately structured to manipulate people into buying them.
I'll put it this way:
If they removed microtransactions, would the in-game economy as it stands be fine? No. So, yes, the problem is the in-game economy, independently of there being microtransactions or not.
We could speculate that
if there were no microtransactions the in-game economy w
ould be better, but we don't factually know that. But then again, games with no microtransactions still have bad economies and require the player to grind, but the real challenge is making the "grind" fun. But that's another debate.
By identifying microtransactions as the problem, even if rightfully so, we are saying that removing them, would solve it, but in fact, it wouldn't as the in-game economy would still be terrible. By identifying the economy as the problem, PD only has to fix it to a point where it's, well, better.
Microtransactions stopped being optional years ago. The "you don't have to buy them" excuse is no longer relevant when time and time and time a-ducking-gain we see how ingame economies shift away from honest and proper balance towards "hey buddy, just get that 5$ coins pack, what do you say"?
also having microtransactions doesn't guarantee "Free DLC". Do I need to even to mention the failure of Anthem, the abandonment of Battlefield V, or endless "roadmaps" developers love to share and feed players empty promises?
They are still optional though. Nobody is pointing a gun at you to make you buy it. That's the same logic of people buying stuff they don't need, just because they are "50% off".
Said that, you can make a point about in-game economies becoming worse, because devs are gready, sure. Bethesda (gready) literally changed the pricing of the Season Pass (or whatever it was called) for Fallout 4, after the game was launched, claiming that the size of the DLCs "justified it". I bought it when it was in a discount for a limited period of time, so I payed like half (or maybe even 40%) of it's full price, and I can say that it was worth it for the price I payed, but not for the full price. They figured the game was a huge success, gotta take advantage of it.
The solution? If the game has microtransactions, still make a balanced economy for it.
And did I say that free DLC was guaranteed? Nope. But given that GTS had microtransactions, and free DLC, that's the best comparison we can make. And given the option, I'd be fine with free DLC, thank you. In an ideal world we wouldn't have neither, but that ain't happening.