Replacing one's ipad with an iPhone Max seems unlikely but possible, depending on what kind of work you do.
For me, 80% of my work requires only MS Office, email, and web. Last year, I installed the MS Office apps for iOS on my 10.5" iPad Pro, and with the Dropbox app I could access all of the files on my "real" computer. Since then, I never bring my laptop when traveling, just my iPad Pro and Smart Keyboard.
The MS Office iOS apps work on an iPhone in landscape orientation. So you could create a smaller version of my travel setup with a Plus/Max iPhone on a stand and a portable bluetooth keyboard. You'd have a lot less screen to work with, of course. For some people, depending on their work flow, this would be a deal-breaker. For others, it might be a worthwhile tradeoff to achieve extreme portability.
When the Max I ordered arrives, I'll play around with this.
But even if it works adequately, I'm inclined to continue bringing my iPad Pro on trips. It's so easy to travel with, being so thin and light, even when I absolutely must travel as light as possible.
The fact that we're even talking about whether a Plus/Max iPhone could replace an iPad (which for some of us can replace a laptop in some circumstances) is a testament to how far the smartphone has come. It amazes me how much we can do with a single device that fits in our pocket - camera, GPS navigator, email, web browsing, photo viewing and editing, iPod-like music player, step tracker, scientific calculator, e-reader, games, watch movies/TV, order tons of crap I don't really need from Amazon, access & view MS Office files on my laptop, catch up on the news or sports scores, and 100s more things. And it's a phone, too.