I think Walter got intimidated by some of the great session bassists passing through the studio from The Royal Scam and on. The same kind of insecurity that Donald had for his vocals. I wish Walter had brazened it out similarly, his bass was so much part of the Dan's aura - you just couldn't legislate for his pauses and the way he'd play with the melody. All the hired guns that followed were industry standard and you knew where they were taking the bass line, not so WB.
I saw them live in the UK twice in the 2VN era and he seemed a bit of a passenger in the band, noodling for noodling's sake. So my feelings for the switch from bass to lead are negative, a road I wish he'd left untravelled.