Would you see an improvement in having a web-based mail client where, with one login, you can access and quickly switch between all these addresses' inboxes?
The only time I use web-based email is when I'm on a public computer and need a very specific message. When that happens, I know intuitively which account it's in.
I can see you're not thinking of it that way. What's your underlying thought on web-based email vs local client?
So far as being in the Cloud goes, all the email is kept in IMAP folders "in the Cloud", I just have a local cache in the Mail app.
Given a high-quality (good UX, etc.) web-based client would be available, would you consider switching?
A native application has advantages over a web site, a web app, or a "native" app built using Electron, etc:
* in resource usage,
* in UX consistency,
* in integration with the rest of the system, eg. for scripting (Apple Script), accessibility, etc
* in working with locally-cached emails while offline or with degraded connectivity,
* in having no ongoing cost,
* in being vendor independent,
* in supporting the use of standard protocols (SMTP / IMAP), rather than a proprietary web backend API
and there's probably more I could come up with if I thought about it some more.
A web application (not an Electron app) has one primary advantage: it can easily access significantly more computational and storage resources than my laptop can. That could be an advantage for searching. There's probably other advantages I could think of with more time here too.
So, while I'd be happy to consider a web-based solution, I think it's fair to say that I think the deck is stacked against such a product, for my purposes.
And honestly, it's just as "in the cloud" as a web based app would be. All my email is synced to my computer from my various email accounts, and that sync is solid and quick enough that I never think about it. I can log into a new computer and add my accounts, and all my email will be there.
I don't have a retina Mac, but I have experimented with a 27-inch iMac in a store: System Preferences > Displays offers a choice of 5 different screen resolutions, but all of them except for the native resolution are IME blurry compared to an Electron app or Chrome (which allows me to change the size of all the elements in the viewport). My vision is not so bad that I don't notice the blurriness even on a retina display.
Graphical Emacs is the only app I know of that uses native Cocoa APIs that allows the user to change the size of most of the apps's elements. (It does not allow changes to the size of scroll bars and elements inside pop-up menus, the title bar and the menu bar, but those things don't bother me.)
I'm surprised you found the options in System → Displays too blurry. How long ago was this? Modern macs have such high screen resolutions that the slightly uneven scaling shouldn't be a problem. Indeed, on many new Macs the default resolution is not evenly divisible by the screen resolution.
On your non-retina Mac, you could use QuickRes to force "retina" resolutions.
I prefer a desktop client because it's more lightweight than a web browser, I can keep it running indefinitely, I don't have to worry about accidentaly closing it and not receiving notifications afterwards. I can also have a dedicated icon in the tray that shows the number of new messages.