Most GTs these days see at least one rider make their first-ever podium - Caruso at the Giro this year and the whole podium the year before, Pogacar and Porte at the Tour, Carthy at the Vuelta. He could definitely fade but it definitely isn't a disqualifying factor, especially as his 2020 Giro third week was stellar and he would have made the top-10 at the 2018 Giro had he not crashed down Sestriere.
I agree he could definitely stay on the podium, and I very much hope he will.
My point was mostly that being good for two mountain stages is very different from being good an entire tour. And it's not exactly uncommon for riders to lose a lot of time in the last week of a Grand Tour. You're right that his crash in 2018 does distort things a bit, as that was clearly his best performance in the GC. But even before the crash his performance wasn't of the level he would need to hold off the likes of Mas, Carapaz and Uran this tour. Again, he could be better this year, and I hope he is, but I don't see these two stages as clear evidence he'll be able to last the entire Tour. If he cracks the 3 minutes he has to the other podium contenders isn't a lot of time. And depending on who is challenging him he might need a bit of a buffer going into the time trial.
I still remember people thinking that Bauke Mollema was finally coming good in the 2013 Tour and make the podium after a good showing in the first 2 mountains in 2013. He actually held on for quite a while, but ended up losing almost 8 minutes in the last 4 stages, dropping from 2nd to 6th. And there are a lot of examples like that, Voeckler lost only 1:08 to Evans (and 2: 22 to Andy Schleck) from stage 10 until stage 18 only to lose 2:25 in the last mountain stage and Alaphilippe list 1:22 to Bernal (and 28 seconds to Thomans) in the first 3 mountain stages after the time trial, only to lose 5:15 in the last two days. If the end comes it doesn't matter how well positioned O'Connor is.