I paid for the game is a very bad argument. I also paid for Skyrim but I don't want to be a sorcerer. But parts of the game are only possible for sorcerers, so the game is locking me out of content that I paid for. I can make this argument for almost every game that gives me some freedom. It's such a week argument.
Very few parts of Skyrim are locked behind a specific playstyle. Heck, you can become the Archmage of the College of Winterhold by beating Ancano over the head repeatedly with a mace and shield if you want to. it's kind of a model game in that respect.
Very few situations in Skyrim have only one solution, even those that have preferred solutions, such as the battle against Ancano for rule of the local school of magic, can easily be creatively solved in a number of ways, and those that are one-solution-only generally come down to you not having worked hard enough in your skills to solve them in other ways. and that's before you even get into a modding community that can never exist for an online community like Elite: dangerous that gives you even more options.
Skyrim has its flaws, but in terms of player freedom, it's a model that FD badly need to learn from.
That said, paying for the game is a poor argument in this case anyway, but not for the reason you've cited. Choosing not to participate in a game mode behind which content is locked is not a valid complaint unless the game mode is somehow toxic or the content required for ultimate success, and as much as there is a coercive factor in forcing people to engineer, it can be done without.
It should still, however, be possible to unlock engineers in more than 1 way. Problems with only 1 solution are toxic in a freeform game. THAT is lazy.
