What do you mean by "emails would break"? They would be rejected by Googles MX servers, and the sending server would relay a bounce message to the sender explaining the problem... That is just a normal function of email.

All MX servers have arbitrary limits. Those limits can be increased/reduced at will without "breaking email"

It is already a common case that people send emails which are larger than the receiving system is willing to accept and are therefore rejected. They then receive a bounce and make alternative arrangements for transferring the file.

The pure market share could have been a motivating factor for parent post. I assume that things behave differently when you're the #1 at something.

However, this seems arbitrary enough to not have that effect.

They would sunset the feature and migrate all attachments to Google Drive, I think they did the same with Picasa and forced a move to Google Photos (I might not remember the product names correctly, I didnt use it but family members did)?