Agreed; most doctrines generally feature a few staples with some abilities like off-map artillery and unit bonuses. For doctrines that have as many as three new units, they tend to be dependent on other units to fight well. Even a Tiger, which is featured in at least four doctrines, cannot expect to do everything on it's own, ad it's at a high cost of 230 fuel. A Pershing would merely be the logical conclusion of the economical demand for quality units- it could also be deployed once at any time like the P4 command tank.
The M26 Pershing in this case would have to be the staple of this doctrine. Everything in the doctrine would be a lead-up to this unit. The unit would likely be weaker than the Tiger or IS-2 to make up for the fact that the US Faction comes with the M36 Jackson.
And like the King Tiger, it's not like it has to be basically the Jackson with the armour of an Easy Eight. The Tiger and the Elefant have the same gun IRL but you sure don't see Tigers firing at something from across the screen, neither the King Tiger.
Hell if anything, a Pershing at the end of a doctrine will likely just end up being used like the King Tiger- super late call-in and often not arrive to the fight anyhow. And yet, the King Tiger is not even doctrinal. I don't even know why they Relic threw it in there, the HQ truck no less. It would be a far better replacement to Jagdtiger, IMO.