An interesting question Kev. While it comes down to opinion, I actually tried recently to evaluate this somewhat objectively one rainy weekend when I had too much time on my hands.
My premise was, if the Beatles had never existed, and all we had was the solo careers of the main three, how would they be viewed today compared to other artists of the era.
I started by looking at single chart success in the US from 70 to 79. Looking at Number 1s, Top 10s, top 40 etc it showed that Paul was up there with Elton John and Stevie Wonder as a successful solo artist in that era. John and George had similar success to artists such as Paul Simon, James Taylor, Cat Stevens and Eric Clapton.
Below them in terms of single chart success were other artists such a Don McLean and Carly Simon
So looking from that angle, John and George would have been significant artists of the era and still be remembered today as such. Paul would be considered a superstar.
I haven't looked at album success yet. That analysis awaits another rainy day. As an indication though, George I think went US top 10 with all his solo albums until Thirty Three and a Third (which got to No 11) in 1976. And returned to the top ten with George Harrison in 78. Pretty decent success by any definition.
This of course has nothing to do with critical acclaim. But in terms of how we might remember them today based on their 70s careers it's a bit of a pointer.