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Answers
There are zero upsides to it. Absolutely none.
You think you'll get all the good things that are usually hogged in a populated world, like rallies and world bosses? All it takes is one guild, or one person with a timer and dedication, to ruin that for you. And trust me, despite the lower population counts, those people exist and love to hog all of that. Sorry, but somebody else had your idea first and beat you to the punch.
On top of that, the broker will be slow. Everything you put up will take ages to sell unless it's lowballed and in demand. This sucks if you're not elite because it'll mean you're stuck relisting expensive and harder to sell items OVER AND OVER AGAIN and paying the broker tax for it each time because there's nobody online to buy your item. You'll also spend AGES trying to buy desired items (and then get stuck paying a LOT of gold due to heavy demand and low supply) while MT and TR will have several pages of that item at an affordable price.
LFG is dead as well, which sucks if you want to do high end content at a reasonable pace. People talk about IMS but it can be a real toss up, especially in dungeons with DPS mechanics like RMNM's second boss or RK's final boss shield phase. You could easily get trapped for hours if you get a group that has terrible DPS, and the IMS kick systems often make it difficult to expel party members, especially if they queued with friends.
If you play on an emptier world, you'll have less people in a game that's based around a multiplayer experience. Don't do it. This is coming from someone on CH. I could lie to you and tell you things are fine so you'd join and we'd get one more person, but I'm not going to do that.