Do Tectonic Plates Merge? I had a question concerning the nature of continental plates.

When looking at the Eurasian Plate today, for the most part, there is one solid fragment of the Earth's crust. However, having had a look at maps detailing prehistoric Earth, this isn't always the case. Throughout the entire Phanerozoic Eon, Eurasia has been a collection of different landmasses gradually merging together. Baltica, Kazakhstania, all the various regions of East and South-East Asia. Yet, today, only India and Arabia are still situated upon separate plates. All others no longer possess any divisions between them.

Why is this the case? I was under the assumption that it was only oceanic plates that submerge into the mantle due their density. I though that when continental plates collide, neither continent submerges, forming mountains. Shouldn't there still be distinct tectonic plates?

Any answers would be greatly appreciated.
Continental collision as Indian Plate with Eurasia one finish in a merge, but the process is slow. An example of merged plates is Iberian Subplate that is presently considered Eurasian Plate.

The collision creates mountain chains as Himalaya. Presently Indian Plate is a separated system because it has still a big granite root under the mountains. While erosion happens, the new geophysical echillibrium bounces new relief to the chain and the root starts to be sutured. There is stil some decenes of kilometers of granite root under the Himalaya, so the chain is still a plate limit.