It's hard to say really. I am sure the community for Splatoon 1 will have been badly diminished by the release of 2, but there are a lot of people who likely have not gotten a Switch yet because they are so rare, who also have Splatoon. Splatoon 1 is still a perfectly great game, if you can get it for cheap, I would highly recommend it. But just a few warnings before you go and get it.
Most of the playerbase will be very skilled at the game. The level cap is 50, and I can guarantee most of the people you encounter will be around there by now. So if you are willing to put up with being challenged pretty hard early on, then go for it.
For the other warning, this is a bit of a less likely thing to worry about, but hackers will likely become more commonplace in that game now that Nintendo isn't paying much attention to the Wii U. They actually just put out a big update to the system which helped hinder hackers for Wii U games in general apparently, but knowing hackers, they will find a way around it pretty soon.