I think IOTA is a very complex project. Conceptually, it could be the game changer for the cryptospace. The problem is they had questionable past. Anyhow, I believe, with the current pace of development, they should be able to deliver most of the promises if not all, but not overnight. I feel like they prioritize better features over delivery in time. Along the way, they might have change a lot of things and take a lot more time to deliver.
As Shimmer is the IOTA itself, the success of IOTA depends on Shimmer. If Shimmer is successful, you don't need to worry about IOTA.
For Assembly, I think it is a great concept as well. But it is a very long way to go since it will be developed on IOTA. If you think IOTA will be a successful project in 2-7 years, then staking IOTA for Assembly won't hurt you, I guess.
Just keep an eye on Shimmer and if it is successful, I don't think you have to worry about IOTA and Assembly.
One thing to note is: Hans Moog, a developer from IOTA Foundation, doesn't seem to like Assembly. Like others, there may be politics and conflict of interests, you never know. So invest wisely.
After all, you are asking an IOTA fan, what you expect from me? :D !LOLZ
!BEER !WINE