If you like either QC45 or XM4 over your QC35ii, then it makes sense to buy it now because it is an upgrade, not "just in case".

U can use your QC35ii as a backup or use it in a rough condition. Many people has more than 1 headphone. Mine included 😁

Understood. It is obviously up to you.

Have you listened to XM4 before?

I have QC 35ii and just replaced my ear pads. Only cost $20. Feel like new again.

Many say QC35ii sounds better than QC45. I have not tired QC45 yet. Not sure if it is true or not. I tried XM4, but I like my QC35ii better because it sounds a bit more natural to me. XM4 has too much bass and is lack of clarity to me.

I would not concern about companies discontinuing their old models. QC45 and XM-4 are extremely popular. You can easier find them even when they get discontinued. Worse case, go to Ebay and buy a refurnished ones. Replace the earpad and it feels like new again.

My point is that there is no reason to buy a new pair because of "just in case" while you are still enjoying your current one. Sometimes better will come out. Wait until you need one and buy the best one within your budget then. You may end up having a better headphone.

just IMHO