Stagecoach has added a few X132 services in for the Fach, running express from Pontypridd Caddy's to Cardiff. Expect they're unlikely to survive long term.
But, as others have said, the rail replacement service is as good as a RRS can be. Combine that with the Rhondda railcard, I can't see enough passengers looking to switch from rail to bus. There's definitely been some switching from rail to car - if you're going to be stuck in Porth-Ponty traffic anyway, if you own a car/have parking at destination, it makes sense. I expect similarly people will switch back when trains return.