I'd say they underestimate how long "just throwing money" (servers) at the problem can work.
If you earn decent money now, scaling number of app servers 10x times to serve 10x times the traffic will still earn decent money. Doesn't matter that "PHP is slow", deal with it when your infrastructure cost warrant hiring more/better developers to fix it.
Especially now. Pair of fat servers on some NVMes and 1TB of RAM gonna cost you less than few dev-months and that can serve plenty of users in most use cases even before any extra caching will be needed.