It really boils down to two things.
1. Is the core game fun?
The game needs to be fun enough on its own to encourage a player to continue playing long enough to unlock whatever features are offered.
2. How much is your time worth?
If you really want something unlocked, and the effort required to unlock it normally feels more like work than fun to you, then you need to figure that balance out. Someone early in the thread mentioned that they could reasonably get 40k coins in about a half hour of play. If it costs $3 instead to get a 40k coin pack, that works out to about $6/hour. If I'm being honest, I feel my time is worth significantly more than $6/hour. Others may feel differently.
And yet, if you read between the lines there, the real issue is how easily this sort of thing can be abused. Make the unlock tantalizing enough, and set that goalpost far enough away to make the task go from "fun" to "grinding", and you will probably increase the pool of people that buy coin packs instead.
The only way something like this works in any fair way, is in an ideal situation, with an ideal development team, that structures a game exactly the same way that they would if there were no microtransactions offered. A steady pace of unlockable content to encourage the majority of players to continue playing the game.
The trick is that no matter how perfectly engineered that unlock:time ratio is, you can only really try to hit the biggest part of the bell curve. You'll never make everyone happy. So, for those that are more impatient, or have less time, microtransactions.
All that being said, this isn't an ideal world, and that sort of balance is at best, a bit of fuzzy emotional math. I can't blame anyone for feeling this sort of thing is foul. It's a system that's far too easy to abuse.
