I don't think that the transition to POS means the end of ethereum. Ethereum already has a very strong infrastructure and many projects are running on the etereum blockchain, it is clear that alternatives already exist today, but etherium still has the highest priority
I do not think so either but they are taking a major risk, there has always been discussions about which protocol is safer but now ethereum is taking the major decision of changing from PoW to PoS if everything goes right then we will see miners leaving for other coins which is not bad, however if there is a single mistake in the process the price of the coin could crash in a matter of hours and that is never a good thing since it could really kill the coin.
The other problem is that after they make this transition then it will be impossible for them to go back to PoW, even if it s possible what will they tell to the long term holders of ethereum, that they made a mistake? That will not fly with them, so while I do not think this will kill ethereum it is really a very important moment in its history that will define how it is seen for the next few years.