Established businesses and/or indirect competitors will most probably not bother getting into a partnership with you, at least until you start having some traction and results. Many startups have the ambition that they will build great partnerships starting from day one and they often get dissapointed. No one is going to do the sales for you. You will have to prove your value first and then ask for permission for partnerships.

Having said that, should you have the luxury of getting a big partner on your side from the beginning and they are really committed in helping you, you have to estimate the ROI of their support to you. But that's a different discussion..

Do you mean 20% ownership in the company? Why would you want to do that? The mere fact that some big company is considering you a threat validates the potential of your product/services. You just need to move forward to transform that potential into reality.

Sustainability could be achieved by being creative in your strategic process and model. Traction should be build with market than competition. Focus on customers and let your competitors focus on you.

If some established company is looking at you as a threat then the ball is in your court. Why do you want to lose the grip by not going solo.

Looking for anything specific? Feel free to reach out with more clarity.