Can be clearance limitations of the frame (as it's the rear that's possible limiting the back). I think the SpeedConcept has listed max 28mm front and 25mm rear. That said, with alternate rims/tyre shapes and taking a less conservative approach on clearance then many are still able to run 28mm.

Could well be the same with your speedmax.

Irony of ironies today, and I know it's been a month, but some specific data for you. I had new latex tubes in for my last race pre injury, March 2020. And so after the bike had been dormant since then I (I've only been using the road bike) dragged it out between Xmas and New year and along with some other things inflated the tyres. After a couple of days I checked and they were still up, did a session on the (wheel off trainer) a couple of days ago with the front and that was fine. Today was first ride outside, so this AM topped off the air. Front was fine, but the rear popped around the valve as I was putting the pump on.

So I'd say 4 years is the point that you are 50:50 going to have an issue with old tubes. FWIW then the front stayed up fine throughout the ride. Handy as just after I got going I realised that whilst I'd got 5x CO2 and 3 spare tubes with me, I'd forgotten to put the CO2 adapter in the bag. Ooops, so 90km of pushing my luck, but got away with it.