Uniswap are huge as a platform at this time, with total value locked around 3.4b$ and daily volume 1.62b$. The only way to shutdown it and loss the value of $UNI is if theres security leaked which enable someone hacked the platform.
It is not because they have a huge daily volume that they can't turn into a scam. Not saying it will happen, but there is always a chance....
I think polkadot is doing great and has good backup though $DOT doesn't have much fame but with time it will grow big and let's see what we come up with by the end of the year. Uniswap! I don't see any lack in the project surely it will survive but coins you mentioned has good potentials.
Polkadot is already in the top 10 on coinmarketcap so they are getting a lot of attention. With the launch of the parachains, they will attract again the headlines, which will hopefully lead to a new ATH.