I don't think this applies anymore like it did years ago when DP was in it's "most active" stages. Games nowadays are difficult to learn. People usually start games with friends and learn together.
Look at fornite, basics are easy (like DP) -- but to be really good its extremely difficult and takes many hours. Same can be said for Overwatch, League of Legends (have to level up to 30 just to do ranked, which is a lot of hours, not to mention the learning curve of getting high elo).
What you said is true about nowadays games. But for DP, you could also see this from another point of view: When people click on "Play" for the first time, they probably see around 20 players online, if not less. So from that first impression, they might already think no developer/admin cares about the dead game anymore, which is why it is full of hackers ("FLYHACKER SPEEDHACKER WTF"). That's what a friend told me, when I showed him DP.
From nowadays games, people know what they are able to do in their game, just by looking at the startpage of Twitch or YouTube. Just take Fortnite as an example, both Twitch and YouTube are loaded with videos about it. Compare that to DP - I doubt, people would look for a video of DP when they just did a ragequit after getting one-balled from the sky, which is why they might not understand that there is a pretty big learning curve in that game. IMO the possibilities of jumps and movement should be shown at the beginning of the game, in the tutorial.
I might be completely wrong, but I think that this is one of the (obv many) factors for new players to leave the game as fast as they installed it.