Why not both? Building fewer houses artificially constrains the supply, driving prices up. Investors prey on the market opportunity. We should definitely build more (IMO "missing middle" walkable mixed-use housing), but we also have to deal with the strong, moneyed, vested interest in keeping housing costs artificially inflated. This literal rent-seeking behaviour is so lucrative they won't just let us fix the problem without a fight. And even if we started to build dramatically more housing today, it would take years to catch up to market demands. So it might be a good idea to get the investors out of the game sooner to take some pressure off the market faster. And not just investors: individuals hoarding vacation homes and short term rentals should be disincentivized, too.