A home 3D printer capable of producing compatible bricks won’t be around for a long time. However, what’s stopping thousands of independent online brands from using high end printers and operating through Amazon, Etsy, surviving all these years later, I think Lego is going to have a lot of bugs to squash. I know it’s a rough comparison, but we didn’t think the music industry would get carved out by independent artists in bedrooms. Because Lego has a Disney/Nintendo brand leveled profile, they’re always going to be the highest quality.
I can’t help but predict a thousand no name companies from China manufacturing and selling fake legos on AliExpress.
For example, take a look at the Free Universal Construction Kit which includes adapter bricks so that any of the following 10 different construction sets can connect to one another: Lego, K’Nex, Krinkles, Tinkertoys, Zoob, Duplo, Fischertechnik, Gears! Gears! Gears!, Lincoln Logs, and Zome.
https://3dprint.com/9147/3d-print-adapter-lego-knex/
Resin printers can also produce these kinds of bricks, and they’re going to be even better.
The trouble in both cases is volume. It’s time expensive to print these sorts of things. There are already half a dozen Chinese companies selling knockoff brand Lego sets (“Lepin” and “Legi”). You can find large lots of used bricks on eBay for cheap. It rarely makes sense to print a Lego brick unless you’re doing something custom.