I’ve regretted selling a few bikes, but some of the changes were effectively forced upon me, like my BMW R1100RT, a fantastic bike but too heavy eventually to move around my garage at that time.
A more recent regret was the sale of my Honda CB500x. It was all kitted out with Honda luggage and Honda heated grips. A great bike, totally trouble free, went really well for 47bhp, sipped fuel, comfortable and capable of cruising at 80. Looking back I must have been mad to sell it.