Given the pace of changes in electronics, it’d be interesting to know how old the OP’s ovens are. And whether they’re the same vintage and model.
Modern high quality appliances can last a very long time.
Sorta true for certain types of caps, .e.g. tantalum. But definitely not true for (the very common) consumer-grade aluminum electrolytic caps. High ambient temperatures and/or self-heating due to ESR will cause the electrolytic paste to (slowly) dry up, which causes the capacitance to decrease over time. For audio applications, a tell-tale sign of the power supply’s electrolytic caps drying up is when you can hear a 120 Hz hum in the speaker, and it will get louder over time.
MLCCs fail due to cracking (from board flex, shock, vibe, etc.).
Film caps are very reliable. But they’re expensive and have very poor volumetric efficiency.
The control board for a consumer oven has to be just about the worst case environment for long-lived but also designed-to-cost-cheap electronics.