The Dem Party will not just fade away; it will be replaced and the GOP will still need your support. And a faaaaaaaar right party will NOT have any more of a chance, then, than it does now.
Polictical parties are not immortal. They can and do "die".
Once the Democrat party is no longer an effective polictical force, there will be a "vacume". The Republicans have grown because many Democrats had lost faith in their party and joined them. These does not mean they embrased the Republican party, they have in fact brought with them many of their own ideas. Which is why the Republican party is moving towards the left.
My point is, there are some like myself who only stay in the Republican party because it is a balance to the Democrat Party. I have no choice. If the Democrat party self destruct, I will find a new home, to the right of the current Republican party.
Can this happen, yes. Will it happen, who knows. But there is no gurantee that the Democrat party will remain a polictical force, and in fact there are signs it is breaking up.