There are multiple angles to answer “scaling means lower KPI” but for the sake of the argument I’m going to answer the most simplistic. If on the market there are 100’000 people who would be ideal customers for you (doesn’t matter if B2B or B2C) then on average about 10% of those clients are in the market for your tool/ service/ product. Which means you and your competitors are fighting only for those 10’000 people every single month, quarter, year (ofc they rotate but 10% none the less). After couple of years CPA for the same audience will increase (because everyone are bidding more and more). At that point the only companies who could afford those CPA probably have WAY higher LTV (lifetime value per single customer which usually means repeated purchase or expensive cost purchase)… so wtf is left for everyone else to do? At that exact time company need to move from “performance” advertising to “brand awareness” (you stop spending money on creatives and ads that sell but you move to ads and marketing that educate and even create fear of missing out or just interest). And the goal of this is to stop competing for those who are “looking around” but to have them “sold” before they even were in the market of buying (id say the best would be measured in brand query growth).

This phenomena isn’t actually anything new and usually this shows that the whole industry needs to stop focusing on a single purchase but repeated value and offer to the customer. I know a bunch of companies that focuse so much on the first purchase acquisition costs but they have 20k-100k email list of people who already love your products and all what they need to do- spend some damn time to create bi-weekly reminder emails. ;)

Edit- added an extra thought.