But that implies that being "extremely well tested, older technology" is an afterthought, not a conscious choice. In other words, if brand-new hardware were magically radiation-hardened, they wouldn't disqualify that hardware for consideration by dint of being new. Especially since newer hardware tends to be more energy-efficient than older hardware, which is a very important consideration for these missions.
And no, this isn't implying that they're going to launch a mission without any testing at all. But spacecraft are specced and designed years before they actually launch; if you want to launch a craft in 2030, you need to pick your hardware in 2025. The intervening years are plenty of time to gain assurance in the reliability of the hardware, regardless of whether that hardware was released in 2025 or 1995.
No, in 2025 you're going to be picking from the hardware that's been already approved for space flight at that point. Reduces likelihood of discovering several years into building the probe that you're going to have to redesign everything to work with a different CPU.
Sure, and by the end of that process (testing, construction, launch, coast, arrival), it winds up being "extremely well tested, older technology".