While later seasons would focus on Homer, Bart was the lead character in most of the first three seasons
I’ve heard this argument before, that the show was originally about Bart before switching its focus to Homer, but the actual scripts only seem to partially support it.
Bart accounted for a significantly larger share of the show’s dialogue in season 1 than in any future season, but Homer’s share has always been higher than Bart’s. Dialogue share might not tell the whole story about a character’s prominence, but the fact is that Homer has always been the most talkative character on the show.
My reading is that it took a couple seasons for show writers to realize that Homer is the funniest character and that Bart’s character was too context-sensitive to be consistently funny. It took quite a bit more time before merchandisers figured that out, to the extent that they ever did.