You don;t have to imagine, you can already see that altcoins will take over.
Bitcoin has already failed to scale and the longer it continues to fail to scale the more altcoins will take over.
If it takes much longer, the damage will be irreversible, assuming it is not already too late.
That is one possible scenario. But it does not mean Bitcoin will die. It will always be there as a store of value. A kind of "digital gold" which is something how, most of the people who are hoarding it, see it anyway.
I completely agree. Bitcoin will never die because has network effect. That means has a big user base because people want it. Therefore, maybe people use some altcoins for everyday micro payments like as litecoin but most of their funds will be stored in bitcoin.