Then we'd have 300 of /a/'s 'gaiafags' and 300 of /jp/'s "I want to move to japan" idiots, all together and vying for board control. It's not like they'd leave. It'd get Touhou all over our anime & manga and Bleachruto Eater all over their readan gaems. Plus, anyone that browses both should probably be aware of the advantages of having more space for two different aspects of an interest.
tl;dr it would be shitty and your memories of "old /a/" are flawed