Depends on the demographic.
Casuals - This could be people who have never played Halo 3 before either due to owning a different console or the Xbox One being their first console. This can include players who just want to hop on, play a few rounds and not care about the competitive ranking system or game mechanics such as BR consistency. These players will likely play on the Xbox One as well due to the game being 1920x1080 and 60fps instead of 640p and 30fps.
Competitive - These players can still be divided into two sub categories, those who prefer the original game and don't mind the low resolution, framerate and prefer the games hit detection and then there are those who can't stand 30fps for a shooter and prefer the higher resolution, quicker response time and lower input lag that 60fps brings to the game. The inclusion of dedicated servers also may be a factor, as Halo 3 (360) used P2P and often if there was a North American player in the game, they would have host connection. Dedicated servers helps to connect players to the closest server, reducing the latency between players. These players are also more likely to stay with the version of Halo 3 they play.
We then have to consider other factors:
Low Player Count - This can affect playlists in OG Halo 3. For example, one reason I don't play Halo 3 anywhere near as much is due to the lack of population in some of the ranked playlists such as Team Slayer. This could be the same for other players and currently with the MCC, there are more playlists to choose from with quicker search times, allowing more variety in the Halo 3 experience compared to the OG version.
Accessibility - This links to the player count. MCC contains other Halo titles and people may pick up MCC just to play their favourite title such as Halo 2 since it's the only way to play that game online through official means. If a consumer is buying MCC for a particular Halo game, they still have access to Halo 3, which means that the MCC version is readily available for the player to quickly hop on and try it out. It is unlikely they will then go back and purchase the 360 original so they may stick with the MCC version if they like it. Unfortunately, Halo 3 360 does not have that luxury since it is not being supported anymore.
Halo 3 for the 360 is still a great game and it has survived far longer than other console titles, much like Halo 2 before it did. Unfortunately, due to some of the factors above, it is possible that Halo 3 will continue to slowly decline and players may shift over to the MCC version as a result.
To be fair, Halo 3 on the 360 could receive a population boost but it would require 343 to update the game one more time and market it. An update to make the game use dedicated servers could help but given the already low population count, this could make things worse, so marketing may help draw players back to the game. A playlist update may also be beneficial to help keep the game feeling fresh or even forcing DLC maps to be installed now to play any of the playlists in order to increase map variety. Unfortunately, none of this options will realistically yield any financial gain for 343 and Microsoft and to add to it, Halo 3 uses old networking tech so it's unlikely any of the above will be able to be implemented.
Halo 3 on the 360 is still my favourite way to play the game but sadly, as with most products, its decline is inevitable.