I don't think it's a big a difference as that. Looking at the traffic counts on SABRE Maps, the M6 Toll seems to have about 30% as much as the M6.The M6 Toll Road exists alongside the M6, but has, I think, 90% plus less traffic on it!
There's probably a lot of traffic on the M6 that couldn't divert to the M6 Toll even if the tolls were abolished, because the M6 goes through more built-up areas and is probably much more used by shorter-distance traffic.