a) you really need to check out some case studies about things like this because people absolutely do not shift to a "better" or even "more complete" product. There is a thing such as lock-in and it's caused by many factors, some of which the user don't even have control over and
b) Consider that people using the other offerings could use your simple less complete offering but which has that 1 killer item (and a couple more as time goes on and you add them slowly). I have seen it many times and I'm sure you have too -- users keep using Photoshop but use that other editor for something special. Or use Office but use some other tool for their specialized offering.
I really think this is the way to go to a) have success and b) even get funding. It's incredibly difficult/rare, even for a team of multiple highly proven individuals, to get that kind of funding and green light to go do a waterfall project based on your hunch that everyone will switch if something just existed.