There will be fewer companies able to afford ads to drive installs for their apps.
IDFA trades privacy for market efficiency. It's used by ad networks to allow me to optimize for either the cheapest or most valuable installs, and validate that my marketing spend is actually driving these behaviors with an acceptable ROI.
Without it, ad networks will be much less effective and it will cost developers at least 2x to drive the same number of installs.
IDFA has obvious privacy flaws, but Apple is making a mistake by crippling it without offering a privacy-safe alternative (and several of those have been proposed by the industry).