In the short term, probably more revenue. But once they race to the bottom of deliverability rates (like SES, for example), MailChimp will forever have lost it’s major defining feature: emails that actually make it to the inbox.
In the world of carrier-grade email providers, they outsource spam scoring to one (or more) of a selection of about 3 vendors (Symantec, CloudMark come to mind). These systems also score against the sending domain, among a number of other factors. Source: my best friend is a lead engineer at a carrier-grade email provider.
And playing whack-a-mole with changing sending domains is ill advised as that will lose the benefit of any whitelisting occurring client side.