If we dig we can find exceptions...
Roxy Music were great but seemed to suddenly run out of steam (or out of drugs). Bryan Ferry was at least as successful, was he not? Though with a totally different genre.
Annie Lennox continues to do all right, although I found the Eurythmics more likeable. Same for Sting. To an extent, Paul Simon. Diana Ross? George Michael? Cher? Michael Jackson seemed to do better once he left The Jackson Five. Was never that keen on Phil Collins with or without Genesis, but Peter Gabriel seems all right, great songs and production. Beyonce does ok now and everyone's forgotten who her girl band was. Lou Reed's ok, though not the phenomenon that were Velvet Underground.
There's a few greats.... Neil Young, Eric Clapton, John Mayall, and back up a bit, Marc Almond.
But the general rule seems to hold in most cases. (I'm ignoring singers that got a start in some unknown band before making their own career). Or Morrissey who is just as depressingly obnoxious but cleverly successful with or without the Smiths. A sort of antithesis to Robbie Williams. Iggy Pop, an icon and almost synonymous with the Stooges.
It struck me with Oasis. A lead songwriter-singer who was good, but thought that was all that was needed, and forgot that what people loved was the band. Probably an unavoidable split. Fleetwood Mac deserve some credit even though their spin off solosist never to came to much, they managed to turn the band's internal angst into one of the best albums ever made. Stevie Nicks made one of the best female ear-splitting songs ever to explode from the best speakers money can buy.... but after that?? not an awful lot!
tbh I don't completely keep track of who's in what band, just if I like the song.
Good post.

Yes Ferry was successful solo and some people I know weren't aware a number of the songs on Roxy Music best of compilations are actually Ferry solo hits. Annie Lennox has one of the best female voices of the last 30 years in my book, Sinead OConnor has THE best voice or did for years and could sing many different genres of music. What I haven't rated is how consistent the albums by Lennox have been, hit and miss for me. Like you I prefer her Eurythmics songs especially the hits which are real belters. She also looked amazing back then, they had showmanship.
Sting has been at least as successful solo as when in the Police and for a long time, I am at this moment listening to his brilliant jazz infused All This Time live album recorded on 9/11 and the attacks totally changed what he had planned for the concert and album. Again though some of his solo albums are hit and miss for me, but so were Police albums.
Paul Simon never reached the huge mountain of Simon And Garfunkel my fave ever group, he hasn't the best voice unlike Garfunkel did but was an excellent songwriter. Graceland was his peak solo for me.
Diana Ross did very well solo and George Michael I rate as THE best British singer of the last 30 years and he vastly exceeded his Wham hits when he went solo, and made some of my fave albums ever like Older which is a masterpiece, Songs From The Last Century and Faith. He was also fantastic live, that man could sing. I was gutted when he died a very troubled bloke.
Cher exceeded her Sonny And Cher fame and hits solo but has always been inconsistent. And Michael Jackson obviously massively exceeded his Jackson Five fame and hits solo, but then not many singers/groups can match his number of hits and record sales, Thriller still being the biggest selling album ever I think. I prefer Collins to Gabriel solo and in Genesis, for me when Gabriel was in Genesis it was all pranching about arty farty with songs 20 mins or more long. Give me their 80s output all day long and I like many of Collins solo hits. He was a really big headed wanker in the 80s but fuck me he shifted records.
Neil Young I love but not with Crazy Horse as its too loud for me, I much prefer his acoustic versions and never more so than his MTV Unplugged, if it had Old Man on it it would be the perfect live album, the problem with Sugar Mountain and some other of his live albums is he talks far too much to the audience. Crosby, Stills And Nash were even worse and its cringeworthy some of the shite they spouted.
I love some Clapton stuff but he has always been inconsistent to me but very successful. What Robbie Williams has or had was a good stage persona, he isn't the best singer at all but he put a lot of effort in live, again his albums have always been hit and miss but some great hits. Oasis were fantastic for their first 2 albums but never reached that level again, Liams vocals were great then, as you probably know Noel only started singing when Liam refused to do so.
Fleetwood Mac from when Nicks and Buckingham joined scaled the heights and Nicks only joined as Buckingham insisted it was both of them or he wouldn't join, he held out for her his partner at the time. It all went to shit with them and all the rest of the band on a personal basis, but they got bigger and bigger peaking with Rumours.