Another issue is that enabling server-side mods means players don't have to buy anything at all. I didn't have to pay for the mods, but even if I did, me paying once would be far cheaper than players having to buy them individually.
I had all sorts of things on my server -- giants, creepers started fires, skeleton arrows could blind you, spiders could poison you, zombies could make you hungry and/or cause the Wither effect, nearby explosions would cause you to become dizzy, there were "space zombies" with glass helmets and 5x the health wandering around (in the Nether too). Monsters would target you from 27 blocks away rather than 16. There were Elementals, invisible monsters, and flying carpets. I also nerfed the enchantments to reduce the power differences between well-equipped and just-starting-out players. That way I didn't have to crank the difficulty up quite as high, and the n00bs could live a little longer. There were shops, and there was an economy. We had mcMMO. We had trading posts stocked with villagers. We had minecarts on the backs of bats, so you could ride in a random aerial pattern if you felt like it. We had bouncy blocks that would catapult you into the air. I added drops (for example, blazes would drop quartz, and magma cubes could drop regular slimeballs). One of my admins made uncraftable blocks such as circle stone and packed ice expensive but available through stores.
The effect of any one of these mods was minor, but taken as a sum, they made up an environment unlike any other Minecraft server. What was the player required to do to enable all of these changes? Absolutely nothing. Just sign on and play.
This is anathema to the DLC business model. Therefore, it can't be monetized by the company producing the game. Mojang was OK with that. (I wasn't running Pay-To-Win.) Microsoft most likely won't be.