But like, let's say you paid 200k, paid off 50k, and now it's worth 600k.

You sell, pay 150k to the bank, and use 450k to pay on another 600k house.

You're new mortgage is for 150k.

I just don't get where this came from:

At today’s rates I’d pay nearly 1.4 million over the life of the loan

Because you're talking about a whole 600k loan. Which makes zero sense given what you're saying