The definition of 'open platform' in terms of hardware primarily means the ability to load your own software, not on how many locked down options there are on the market.
The fact is, the majority of systems on the market now are closed. Examples include game consoles, cell phones, tablets, and embedded machines in consumer products of all types. The only exception is the desktop PC, and even there, many are now shipping with UEFI locked bootloaders, and we have only the 'magnanimous' promise from Microsoft to continue offering their signed stubs for use in loading alternative OSs. Initially, the ability to shut off signed loads was mandated, but MS has now changed that policy. It's really only a matter of time before most PCs are locked down with the OS they shipped with. Today, the open system is the minority, not the majority.
Popularity is often not a good measure of quality, openness, or freedom. It's just a measure of dominance. Youtube is dominant because it was one of the first, not because it is now the best. Even if it was the best when it launched, continued dominance does not imply that it continues to be the best.