Aren't they already? At the top of my inbox is a message informing me that I'm almost out of space. My infinitely expanding mailbox limit from Google has stopped expanding. A good things come to an end.
Gmail's free quota is more being quickly depleted when Google started counting space used by 'high quality' (limited to 2000x2000px) photos. Previously that was free, but not any longer.
What I find interesting is that I spent a good few evening clearing out the junk from my gmail account. I got it down to around 80% and within a few weeks I'm back to 95% again...
Photos backed up from your phone count as part of your Google free storage. I had far more space consumed by trashed photos that had been backed up than by email.
You need to unsubscribe from things you have no intention of ever reading and start filtering crap you don't want straight to the trash.
Gmail has a pretty good interface to creating automatic filters to tag,move,or delete crap built in or if you prefer imapfilter lets you basically write a lua program to filter your email.
I'm pretty much awful at maintaining my inbox. I don't know how people do it. There's just so much junk and I get like 1 important email for every 5000 unimportant ones.
I guess that's what filters are for but I'm pretty lazy in that regard too.
Take a day to religiously unsubscribe from trash. it's likely that over the years you've signed up for many news letters (because of shitty dark patterns) that keep spamming you.
All those emails are required by law to include an easy unsubscribe link in the email, usually it's at the very bottom in a small font.
I found that my email has the same problem my real world has: it isn't that hard to keep stuff organized when it is clear where each thing goes.
When it is not clear, or there isn't a good place to put it, stuff starts to pile up.
Once I have figured out where it should go, which is 99.99% of the effort, actually getting it there usually isn't that big a deal. You can organize 500 emails in an hour if you have set the systems and the shortcuts up - less if you can do it with filters.
No one weird trick, just the obvious. Don't sign up for too much, strictly limit your inbox to one page, delete or handle+archive as necessary to achieve that. I've been using Gmail since 2004 and am currently using 0.54 GB of my 15 GB quota; I've never regretted deleting anything later.
When you say "so much junk", are you talking about spam that Gmail isn't catching? Or just legitimate mail you don't find very interesting?