There are some units of WAP1 which were 'upgraded' to WAP4 and again, reverted back to WAP1. Maybe those units were more 'volatile' as WAP4's. Example - WAP1 22029.
Regarding P7's, they too had limited usage earlier - mostly with the premier classes like Rajdhanis & Shatabdis. Eventually, they've emerged as more powerful alternatives to the P4's. Nevertheless, WAP4's have been the "most successful" passenger locos so far!