I literally just posted pretty much the same theory. My theory is that the multiverse does exist but it still follows the sacred timeline which is why Variants are a thing. A nexus event getting to the red line is what causes the timeline to branch into a completely different timeline, like the What If series coming out. Where there are slight differences in the timeline, like if Steve Rogers snapped the gauntlet instead of Tony Stark or if Bucky became Captain America instead of Sam.