I've been back and forth on this. It's a tough one. These are the options (without getting into anything to fancy):
1. Pioneers return to their original costs. This incents players to quickly produce and spread pioneers for a Pioneer Explorer strategy, which can be fun but I feel like it is an unbalanced decisions and players don't have the play in the single city model for long.
2. Increase the labor costs of pioneers. This just slows things down, i tihnk this probably the worst option.
3. Gate Pioneers. We could have pioneers unlocked with a later civics tech. I've always been interested in this idea but I've never found a way that made it fun. Instead you lose one of your early growth strategies. You either ignore it and play as a 1 city game or feel forced ot rush to that tech every time because the civs that have it get such an advantage.
4. Pioneers cost Pop. I like the idea of this solution the most. But it comes with a really high cost, these kind of resource decisions are very hard for the AI to make, so the AI ends up running on the tightrope between being to conservative (and not expanding enough) or to aggressive and pop starving its cities. So this becomes unworkable because of the Ai considerations (gating pioneers behind a tech is also a difficult mechanic for an ai that isnt hard coded to deal with).
5. Pioneers cost Gold. This is our best compromise that ties pioneer creation to the economy, causes players to more carefully consider when they build them and is the easiest for the AI to deal with.
So that's the logic. I can't say any fo the options are right or wrong. But thats my thinking as I considered the issue.