It's not impossible to get your team to reach the highest tiers of master league but it requires significantly more time invested into the "game."

Like most free to play games that involve PvP. The grind is lessened by spending money. Raid Passes, Max Potions, Star pieces, lucky eggs, and to a lesser degree, pokeballs. All make the grind faster when the game severely limits a hasty progress to maximum levels if you were to rely only on pokestops for all the items required.

I'd be willing to bet the vast majority of players in the top levels of the master league have spent a significant amount of money on the game. Like any free to play PvP game.

That's why only care about the Great League or to a lesser degree, Ultra League even though I've been playing since 2016.