Everybody agrees it's a good thing, that it's something we should be concerned about in gentrifying neighborhoods, but people are rarely very clear on what they mean other than "build housing that people can afford." Well, yes, ok, but...
The cost of new housing, as opposed to the price, is roughly land acquisition+construction costs (I'll leave out financing for simplicity). If housing is priced below that, someone has to eat the difference.
The price of housing can certainly be higher than actual cost, particularly in places with rising land prices. People who have been sitting on empty lots that were once worthless but are now in gentrifying areas are getting a windfall. That is, their land acquisition costs are minimal but the price they can charge, reflecting the current market value of the land, is much higher.
Providing more affordable housing means building cheaper units (smaller, multi-family, denser up to a point, fewer amenities, no off-street parking), or subsidizing them for people, or both.
The gentrification wars in Philly are just weird, and well, mostly not what they seem. There isn't a big push for building cheaper units, there's just a desire to block development. And, yes, mandate parking when development does happen.
Philly isn't San Francisco. Here, building more is the most straightforward way to lower prices/rents. Sure building more can speed very localized gentrification and land price increases, but generally the way to respond to demand increases is to increase the supply.