Where does the old generation end and the new one begin? Alongside the older GT winners like Nibali Thomas and Froome, you have the likes of Aru and Quintana who are both 30 but with their best years behind them. Of the recent grand tour winners, there's Roglic (30), Dumoulin (29), Yates (28), Carapaz (27), and then the "youngsters" like Tao (25), Bernal(23), Pogacar (22).
Also, if you look at the list of grand tour winners, they seem roughly split between people with brackets after their names (showing which of the multiple wins that was), and those with no brackets signifying that this is their only GT win.
https://en.wikipedia.org/wiki/Grand_Tour_(cycling)#Grand_Tour_winnersA few of the guys I named are going to end up as single GT winners, but the only one I would put the house on winning another is Pogacar.
Edit - the guys I feel slightly sorry for are the nearly guys approaching their 30s. Bardet (29) has come second and third (and 3 other top 10s) at the TdF (and second at the worlds). Thibaut Pinot (30) finished 3rd at the TdF aged 24, and hasn't cracked the podium at another GT (4th at the Giro in 2017). Mikel Landa (30) has a third and two 4th places in the last few years since he got to lead a team. Superman Lopez (26, so not quite so old) had two third places in 2018 but seems to have gone backwards the last couple of years. All of them would have thought themselves ahead of Yates and Roglic a couple of years ago, now you have to wonder if their opportunity to win GTs has gone.