Do Tectonic Plates Merge?
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.