I'm not sure that would clear up all the uncertainty, MK. In this case, Morbius had played baldie successfully, but he didn't necessarily know that baldy is an allowable word - he couldn't test it out as that puzzle had no y. More broadly, if we implemented your suggested rule we couldn't assume that every player would know about that rule.
A lot of such issues would disappear if we excluded all plurals ending in s. In other words, no baldies, but also no babies, not to mention no boxes, no potatoes and no indices. Maybe it would have been better if I had started with that policy, but to change to it now would drop out thousands of currently accepted words. In any case I think those types of plurals present an entertaining part of the challenge.
For quite some time I've been refraining from making any changes relating to "plurals" issues, feeling that I'm likely to do more harm than good. As the passage from 2007 quoted by Ozzyjack shows, I was already apprehensive 13 years ago, when I added baldie and left baldies as an accepted word. If only I'd let sleeping dogs lie then!