I definitely feel your pain and have, like you, chosen to add a comment rather than just click on I Also Have This Problem. I think voicing your dismay/disgust via Apple's feedback mechanism is really important. They do listen to user feedback and will change things if they think it's good or what they did wasn't the right direction to take (except that you never know what the impetus actually was ultimately).
But in thinking about this mechanism, that is, the current lack of it, I don't think their motivation is to get customers to purchase phones with more memory to accommodate email storage. Most people just aren't going to think that way and wouldn't even know to look in Settings > General > Usage > Manage Storage to even know how much space their email is taking up.
What would be a better solution would be to have a way for a user to select how much storage to allocate on their mobile device, rather than number of emails. On mine, my emails are taking up 2.6 GB of space on iCloud but only 388 MB on my device, not enough to buy a new device, LOL! I'd rather say, just allow 250 MB of space to my emails and don't back up my Gmail email (as a separate setting). Then, "Manage Storage" in Settings would have some meaning. What's weird is that under "Manage Storage," the allocation for Mail is the only one that can't be changed!
There are multiple issues with doing even that. If, for example, your iPad is your primary device and your email is set-up as a POP account, then going in and limiting the size would by necessity delete emails, which would result in losing them forever. Not good. If, alternatively, all your accounts were IMAP, then presumably you could limit the space on the device, knowing the server was keeping your mail safe. (In your case, you have a desktop with plenty of room so you likely have a local copy and probably just have your devices set as IMAP clients; at least, I'd recommend that approach.)
But if you want a conspiracy theory, a better one would be this: If you're backing up to iCloud, and you have multiple devices storing gigs of email, then you'd be more likely to up your storage, thus paying more to Apple for storage. But if your email is already iCloud based (.mac, .me, .icloud) then it's not (as explained here) getting backed up anyway and not taking room in the cloud. It's not clear if your non-iCloud based email does get backed up, thus using more iCloud storage.
I don't think Apple would have a plan to manipulate or trick people into buying more storage on their iOS devices using email as the mechanism. (Another conspiracy theory: Just keep upping the memory requirements of iOS itself, making the devices with less storage slow down to a crawl, and there's your motivation to upgrade.)
Whatever else you say here, take a moment to voice your opinion to Apple directly.