Community is a very large reason why many people stick around, if you turn up with a group, you're much more likely to stay, there's natural competition between members and the experience becomes more chilled. Problem is that a large majority of people who join are going to be coming in on their own, I'm with Ftx on this as my experience was similar to his. Mainly played ladder up to 2016 where I found the GB lot.
People don't usually want to improve to then commit to a game, they will improve because they're already committed, and that's why training material won't resolve this issue. Again more material is good, just doesn't help with this issue. The factors bringing people in are doing extremely well, Yuri, Brnk, Gyle and many others have put some serious hours into the community and to that they have my thanks.
Retention is a completely separate issue, you don't leave a game because it's "too hard" or because one person called me a smurf, you leave a game because either the community is the issue or the game isn't fun.
Now unfortunately we can't dictate what is fun to some people so naturally people will leave when the game becomes less entertaining to play, now, usually that doesn't happen for newer players, but since the point is players leaving before 2 years, it cannot be completely disregarded. This will hopefully be helped with the addition of tmm due to less games being dictated during the lobbysim, but the results will not be seen for a while, to see over 500 names on the 2v2 leader board gives me hope.
As for the community, it's not bad. It's leagues friendlier than league, CSGO and other high player count games but that doesn't quite cut it. It can be a little discouraging being a 0 0 to find nobody wants you, but hey that's where ladder and tmm help by removing hosts. Maybe a better clan system would help, possibly categorizing them into relaxed, beginner friendly, competitive, coop groups etc.
What you need to take from this is that no, training isn't the answer to retain players, as most people play the game to enjoy it regardless of how good they are, if any more evidence is needed, refer yourself to the last ladder league where lower brackets had the players with the highest time spent in ladder. What will probably help is a more helpful way to introduce new players, I know #Newbie channel works for people below 50 games but maybe having a newbie channel per major language (Russian, English, German etc) might help people find others in a similar situation.