Whilst I concur with your thoughts on experience, he isn't even the best 6 in the club right now.
The harsh reality is that he's lost his zip. He isn't the player he was for us. Lomax is a good half in a style that involves using the halves in the middle third of the field, but we don't play that way under Woolf. The halves are pretty much starved of possession until the area close to the line. Without that ability to accelerate from standing still, Lomax is continually nullified.
It is possible that we may not have lost against Catalans if Lomax had been playing, but I could equally counteract with the fact that we wouldn't have scored so many points either, so there is no definitive answer to this.
I do tend to share the same concern about big match temperament, though, and that is why I can see the alternative view.
In all honesty, perhaps Welsby and Lomax for this season's big games would be the sensible pairing, but this comes with risks and those risks are playing a partnership with no experience of playing together, and playing a player who has looked less than 100% right for a long time, without having played more than a game in a few weeks. Have we not learned the lessons of 2019, when playing players at Wembley based upon reputation and experience rather than form and fitness cost us dearly?
As I said, if we lose, there will be blame attached to the half back combination, whatever it is, and people on here will be giving it large and getting abusive towards those that held the alternative view. That's actually why I'd have preferred there not to be a choice, and the same would apply for Fages, because undoubtedly there would be some clambering for him to be included instead of Jack Welsby because of his "experience".
Ultimately, regardless of who you think is the best player, playing half backs who aren't match fit or match sharp in big games nearly always finds them out, and is one game in six weeks going to be enough to get him back up to speed? I'm just not seeing enough from the 2021 version of Jonny Lomax to make his selection justified as things stand.