But the mojang account was fine? That's my point, they already have your email address.

A lot of us(longtime motivated Minecraft players) bought our first accounts when we where fairly young, and often used emails from parents/school, which we no longer have access to. Another reason why this is a huge deal is that this means Microsoft terms of service apply. Also this means that the random account banning that can be experienced with Microsoft accounts will come to Minecraft.

Not only that. I had used a normal email address and still had to contact support multiple times for that account transition a while ago. (Or where there even two?)