Romero has also played week in and week out. He has more appearances for his national team than Dean has in Championship and EPL. He has been reliable for us, and for Argentina. He also has played a World Cup final and a few Copa America finals (though he has lost all of them but not because of him) in addition to Europa Lague final. He is quite experienced in high-pressure matches and has been a good servant, so deserves a shot before Dean.
Probably better would be to have both of them next season and sell De Gea, but I don't see us selling De Gea.
The last time Romero had played week in week out was in 2012/13. To give you some perspective, this was the season when we had won the PL. After that he had lost a starting place in recently promoted Sampdoria and never played as a first choice in club football since.
His international career is quite impressive, although he never got even a remotely decent competition. Still, for the entirety of career there he was always an error-prone goalkeeper.
He’s a fantastic second choice keeper and he has been mostly brilliant for us given the chance but he’s not good enough to be our number 1, he never was and he never will be. His “experience” is very much in question as well. Considering what Henderson had achieved already, there is a possibility that he’ll become United’s number 1 for years, so there’s no real argument for benching him for Romero.
It’s the same as when fans had argued that Lindegaard was a better first choice than De Gea. Some players that perform well in secondary roles should get proper credit for that, but, at the same time, we shouldn’t overrate them – it’s a very specific role and the requirements for it are very different compared to the requirements for a first choice keeper.
P.S. as for the international career, for some context – Marcos Rojo is currently on 61 caps for the same Argentina side and he was a starter in their runners up World Cup campaign in 2014, same as Romero. Heck, he even got in the All Star XI somehow.