IMO it's fine only if it's optional and doesn't negatively impact the experience if you don't play online, like in No Man's Sky.

And what consequences in the "real world" are these?

In the main single player campaign of Assassin's Creed Unity you earn points throughout the game to upgrade your weapons, similarly to many RPGs. I distinctly remember selecting an upgrade once and a popup appearing asking me to pay real money for the upgrade.

It completely threw me off guard as I thought I was playing an offline sandboxed game. This popup killed the game for me as it broke the immersion, reminding me that I am not taking part in the French revolution, just a guy with a controller in his hands and a credit card in his wallet. I stopped playing after that.

I am not arguing against micro-transactions, I am arguing against the blurred line between offline and online.

This has been one of my big issues with microtransactions. There is no reason, in the abstract, that they must be more expensive than buying a game one-off and they could (theoretically) be more fair by amortizing cost across your playtime (and thereby allowing you to pay less if you get less).

My two big issues are that I find microtransactions to be mentally wearying. I play games to get away from life's responsibilities. Microtransactions, by requiring financial decision making, drag me back into the very thing from which I am taking a break.

The other issue is that most microtransactions create a perverse incentive for the game developer. If the consumable bypasses a grindy part of the game, then the developer is incentivized to create a grindy (i.e. boring) game. If it is cosmetic, they are incentivized to create a somewhat bland game. There are other variations, but they all seem to rely on creating a game that is inferior so that I will 'upgrade' it. I'd much prefer that the game creator's interests be aligned to mine.