The bolded part is also how I see it. I feel like in the case of Firmino, you don't even have to replace him with a like for like player to make the team work. A player like Salah is much more valuable because He could decide the outcome of the match alone, the X factor that most players don't have even if They try their best. His quality is easier to measure. I bet He costs more expensive in the market than Firmino as well and that speaks a lot.
If you put it strictly like that, where False number 9's are the only candidates that could replace Firmino, of course there will always be the shortage in the numbers. Why? because only a few teams actually use it. The role is created through experimenting with players so We didn't know if there are more players that could excel in such a role. Has there ever been a player who started their career as a False 9? I'm genuinely curious. Messi wasn't a False 9, Firmino wasn't a False 9. So because of this, We can't be sure if Firmino is actually the only player that can do well in this position for Liverpool. Sure, there are traits that 'make sense' for players to have to be a False number 9. But still, it's all about experimenting. You can experiment with AM/WF/SS/CF.
You can be sure 100% if forwards like Depay are much worse than Salah. But you can't be sure if Son was put in Firmino's position, He will do worse. You can't be sure if Griezmann was put in Firmino's position, He will do worse. You can't be sure if Eriksen was put in Firmino's positon, He will do worse. Nobody knew if Fabregas could be the False 9 in the EURO 2021 winning team.
I feel like Di Maria is a good comparison for this. When He left Real Madrid, He was the only world class player who could play like He did (does this sound familiar?). An attacking midfielder/winger with good work rate, that is also quick as feck, had a wicked cross and capable of beating his defender. Was his best position a winger or a midfielder? nobody cared, everybody was mentioning how Real Madrid made a huge mistake and lost their engine, one who was more important than Ronaldo, Bale, Modric, etc. Guess what? He was replaceable. In the case of Ronaldo however, once He left the team, he impact was huge. And Salah is the equivalent of Ronaldo in this Liverpool team. Not by productivity measure alone but He's the X factor in that team and also Mane, to a lesser degree. Only after them could We start talking about Firmino.