4 of those players you listed were brought in by Bowyer with limited spending, and all sold for a profit. A slightly different approach to Mowbray splashing £12m - £13m on two players - Bowyer probably reached £5m in transfer spend throughout his time at Rovers, with a healthy return of £18m or so based on your calculations above. Equally, Mowbray did superbly well with Armstrong and is likely to do the same with Brereton, and deserves praise for that.
In terms of Hanley - he was an academy player who Bowyer managed to tie down to a new 5-year deal. Very similar to Lenihan who would probably attract similar money (probably slightly less) if he wasn't out of contract this summer.
In terms of Rhodes, Dunn, Murphy, Etuhu, Best, Nuno Gomes, Formica, Goodwillie, Orr etc etc etc etc.... only Rhodes was any use to Bowyer, the rest were either past it or an absolute joke to start with. We all know Rovers were a basket case then, let's not pretend Bowyer benefitted from our insanely high wage structure. Would be wiser to recognise that Bowyer got rid of the deadwood and built a young, likeable squad with good sell-on potential. His overhaul was actually far greater than Mowbray's in terms of squad.
I maintain that Mowbray NEVER had to sell anyone he didn't want to, except Armstrong - look at our outgoings in 5 years... Which of those do you think Mowbray wanted to keep at the club? He didn't even want Raya to stay on. 15 on that list (out of 30) were TM signings, and a further 6 (of the existing squad when TM arrived) signed a contract during his time as manager. Mahoney didn't sign because he got a better offer from Bournemouth, but the offer was there.
So 21 of the 30 outgoings in 5 years, TM made the decision to sign or extend their contracts.... that's fine, his decision, but let's not pretend he's had his back to the wall and has been forced to break up his squads and start again on a pittance. Two, maybe three (bolded) of those players went for a transfer fee - the rest were free transfers or left without a club.
Jason Steele
Hope Akpan
Jason Lowe
Anthony Stokes
Conor Manhoney (end of contract)
Liam Feeney
Elliott Ward
Gordon Greer
Wes Brown
Paul Caddis (his signing)
Peter Whittingham (his signing)
David Raya
Ben Gladwin (his signing)
Paul Downing (his signing)
Jayson Leutwiler (his signing)
Danny Guthrie (his signing)
Derrick Williams (extended contract under TM)
Danny Graham (extended contract under TM)
Dominic Samuel (his signing)
Joe Nuttall (his signing) ((extended contract under TM))
Richard Smallwood (his signing) ((extended contract under TM))
Craig Conway (extended contract under TM)
Jack Rodwell (his signing)
Adam Armstrong (his signing) - Great profit
Lewis Holtby (his signing)
Sam Hart (his signing)
Stewart Downing (his signing) ((extended contract under TM))
Elliott Bennett (extended contract under TM)
Amari'i Bell (his signing)
Charlie Mulgrew (extended contract under TM)
Corry Evans (extended contract under TM)
Reflecting on Mowbray's time as manager - Until the spectacular implosion of last season he'd done a very good job of steadying the club and solidifying us as a Championship team, not forgetting the fairly regular run of 1 win in 8 etc etc. Last season, with the squad he had at his disposal, a 30-goal striker, Harvey Elliott et al, we blew up spectactularly. This season, 8th Feb 10 points clear of 7th place, we did it all over again.
We were punching above our weight in 2nd, but we got there on what was frankly an extraordinary run. Even mid-table form would have seen us in the play-offs from there on in.... Cue 3 wins and 5 draws in the 17 games before promotion was mathematically impossible, failing to score in 11 of those. Extrapolate that over a season and it's more or less 35 - 37 points... a couple more than Derby (who were already deducted 21 points), sat in 23rd in the league. That's how bad we've been since we beat Boro at home!
I can see the argument as to why it might be time for a change, even understanding the enormous risk that comes with letting these people pick the next manager!