Some portion of equipment being out-of-service is to be expected, and the more complex the equipment, the more time it will spend in maintenance instead of working. It could be that for a plane as complex as the F-35, readiness rates simply cannot be that much higher, due to just the inherent complexity of the concept.

But if you notice a very obvious and accelerating downward trend , you're supposed to, like, take some action to deal with it? Procure some new vehicles, procure a whole lot of parts to repair the existing ones, anything! Russia for example is still manufacturing BTR-82s, BMP-3s, T-90s (they're also refurbishing a lot of old stock, especially of tanks, but they do have new manufacture going on too), and have been substantially increasing the pace as the Ukraine war has continued.

However, in the American case, a bunch of the equipment cited here has long been out-of-production, or has had its production substantially down-scaled, and for some of them the US is several procurement programs deep into failing to find a replacement:

• The last Abrams rolled off the assembly line in '91 - there was some continued manufacture of kits which were assembled over in Egypt, but as far as the American military is concerned, it was over in the '90s. Everything since then has been upgrades and refurbs of existing hulls (the exact thing Westerners accuse Russia of doing!)
• The bulk of the Bradleys was made until '95, after which there was continued low-rate production and upgrades of existing vehicles. It at least lives on in the form of the AMPV (which isn't mentioned here), which is essentially a turretless Bradley - except production of that was scaled back by Hegseth this year. They have tried to replace it several times: FCS, GCV, OMFV (which was actually paused and restarted, so it sort of counts as two programs)
• The Stryker was manufactured throughout the 2000s, and is now having procurement stopped and upgrades scaled down
• The LAV-25 was produced in the '80s. Technically, the Stryker is a sort of cousin to it (https://en.wikipedia.org/wiki/Mowag_Piranha#Family_tree), so at least one of the dynasty's branches survived in production... until this year
• The M113 is ancient, having production massively scaled down in the late Cold War and then finally stopped in 2007. It was supposed to be retired to mostly rear less-combat-centric tasks with the Bradley and Stryker coming in, but that didn't happen, and so the AMPV (which, as mentioned above, is essentially just a turretless Bradley) was supposed to replace it, this time for real - but that's also not happening anymore apparently.
• The Paladin is being produced in a modernized variant (at, of course, a slow rate), but most of them date back to the '90s. This one also has a bunch of failed replacements: XM2001, XM1203, M1299, here's a twitter thread going over this
• The ARVs are still being made ( at a very slow rate), but the bulk were made in the '90s and early 2000s, and there's a bunch of even older ones from the '70s too
• The AAV was produced in the late Cold War and '90s
• The ACV-P and JLTV are like the only ones to be recent (at least from the combat vehicles, the logistic trucks are generally a lot newer), but the ACV-P is a relatively niche USMC-specific vehicle, with under 400 in service and a total of just under 600 as the final target, and the JLTV... was cancelled this year (although tbf, this one is actually reasonable - it was a vehicle specifically designed for the IED-heavy COIN operations in the Middle East, and isn't particularly good when pushed into a more conventional APC role)
• The Humvee was mostly produced in the '80s, '90s, and I assume early 2000s to some extent, after which production scaled down. And it was - you guessed it - cancelled this year. Although at least the Humvee, JLTV and Stryker will continue production for foreign clients, but I assume there'd be a lot of down-scaling, especially for the Stryker given that there don't seem to be a lot of takers and the 8x8 wheeled APC/IFV market segment has a decent amount of other options.