As is noted in the article, selling band shirts was not yet common practice when the Ramones starting doing it. Until Napster came along tours were marketing for albums, which were the primary revenue source.

I seem to recall reading that Gary Holt or Jack Gibson, either from Exodus, claim that despite being known worldwide as a thrash metal act they have to support themselves selling t-shirts, since their earnings from touring, albums or streamings won't cover their expenses

Not in the past. When that change flipped from music sales to merchandise and tours, I couldn’t be sure but I’d reckon the early 2000s.

It's not that they made more money from merchandise, it's that they sold more t-shirts than albums. Implying that more people were interested in the "image" of punk rock than the music.