The reason people bring up the "life's too short" answers is that finally, eventually, that is the question you can't avoid and you must prepare for.
In the case that your partner won't conveniently budge you will be faced with this very priority-setting question: "Do I want to do this thing of mine (and let my S.O. possibly leave me) or do I value the companionship of my S.O. more (and let myself pass on this idea)?" The answer to that question is not an ultimatum per se but an ultimatum can very easily be made out of it.
Then, the reason I do bring this up is that you have to have the answer to that question all laid out before you go ask the opinion of your S.O. Otherwise you're really not asking: you're just knocking on the doors at random, in hope for an easy answer.
However, as soon as you have the answer things will, once asked, unfold much easier. One party being clear about his life helps the other party find clarity as well.
If you're certain that you want your startup, then your S.O. will read that and it's his/her turn to evaluate his/her priorities, and it might very well be that he/she will find the supportiveness for your endeavour now that you're dead serious about. On the other hand, as long as you're just toying with the idea, he/she will just carelessly drop you an initial opinion because he/she's not really enforced to face it yet. And people don't like to face questions unless they're forced, at least to some extent.
This is pretty much want you can do: find out what you want and then see how it goes. It's okay to bring it up seriously but still cancel later if you get scared enough. At least you've tried.
What you can't do is change other people. They will change themselves if they feel they want to but you can't convince or talk them into these things. And it's pretty damn hard for them to change if they don't who you are and what you want.