In fairness, and whilst I don't think players should be ruled out when they hit 30, the problem is you don't know when someone is going to be finished once they're in their 30s. There is a fairly rapid loss of pace that everyone gets at some point in their 30s. Sometimes it seems to happen at as low as 31, sometimes it's mid or even late 30s. In a similar way, at some point players will also lose their recovery abilities in this age bracket. So they might start only being good for about half or so of the games in a season. And injury proneness is another factor that worsens.
It can be none of these really kick in for years, and some players do manage to work their game around a loss of pace, but it's fairly rare it doesn't become some sort of liability. Some players can also still be very useful squad players even if they're only fit half the time, but that doesn't warrant paying them top earner wages, which is likely what we're talking here.
It's fear of the unknown, the unknown being when this player will start to break down. And that's precisely why players usually start getting shorter contract offers in their 30s.
Again, it can absolutely work out for years, or it can fail (like yes, any signing), but the degree of risk undoubtedly increases in the 30s.
To be clear, I do want to keep Tronstad and would happily offer him a 3 year deal on decent wages.