If anyone is curious, this is the first step to monetizing email beyond ads. Fast forward a few years and the store as much content as you want (Gmail model) will be an exception not the rule.
When Gmail launched and Yahoo panicked and increased limits, they quickly realized that users actual e-mail storage use didn't grow very quickly - it's probably far outpaced by growth in storage densities.
Shifting attachments to Dropbox gives sufficient space for most users on the free tier "forever", but of course Dropbox gives users other benefits which they might very well want to pay for.