So on one hand yes it makes sense to splurge when money is cheap and then cut back when it isn’t, on the other you may cause long term damage to your business doing this too much. I think tech companies probably overdid it the past few years.
Perhaps that incentive is the problem?
The incentive isn't something artificial, it's just how certain markets work. E.g. a city might be able to support hundreds of pizzerias, but only 2-3 ride hailing companies.
The mythical man-month is about how adding individual new members to an existing software team doesn't necessarily make the team deliver faster (and can even slow them down). Nowhere does it claim that adding more parallel projects or growth initiatives to a company slows the company down.