Mandatory expansions would be *way* worse: you would have to pay for every new content, not just the paid part of the DLC, even if you don't play it. If you don't pay, your version will not be prioritized for bug fixes. You couldn't choose to disable a particular DLC if you don't like a mechanic.
Can you imagine what the game would be like if there was no free content and you had to buy DLCs in the order they were released?
Disagree. The only mandatory (as in considered by the community) DLC which is not well-liked in the community is Common Sense. Art Of War and Rights Of Man were received with huge praise by the community. Art Of War in particular changed the way how warfare and diplomacy works so well, that playing without AoW is a very, very terrible idea. RoM just had so much stuff for everyone (mechanics for Coptic Christians, two unique government system for Ottomans and Prussia, completely new technology system that it has become an essential DLC.
And even then, one of the big reasons why people were outraged by the Common Sense DLC, is because the cost of coring conquered has skyrocketed compared to DLC. The other big reason was because of the cardinally new system of development, where you couldn't do anything if you got events reducing the province's development (like that "towns attract serfs" event everyone gets at least one time per a game) without purchasing DLC.
I'll skip this immersion pack/DLC, mostly because I felt that Third Rome immersion pack had rather uninspired mechanics, which were added simply because the development team just thought, "let's add mechanics which will make Russia OP, and we'll also not balance it properly. Y'know, because we need to sell SOMETHING!" You know what, Paradox? Give me a better combat mechanics DLC, or at least add an option into the free patch to toggle on and off combat width terrain penalty, which would be Ironman-compatible.