Has anyone ever considered that the replacement rate of 2.1 should be becoming lower with longer lifespans too? Or if it refers to working population, probably

Has anyone ever considered that the replacement rate of 2.1 should be becoming lower with longer lifespans too? Or if it refers to working population, probably becoming bigger. I've seen this figure quoted for two decades at least, and I can't imagine it staying constant with all the other changes. Not to mention a clear move towards childbearing at older age, which skews the stats for a while, and also reduces number of children because it's that much harder for mothers to conceive with every passing year. I think demographical models in use are in serious need of updating to be taken seriously.