Yep, keep on thinking, that you're smarter than Intel engineers. Have you though of googling, "Intel processor failure - TIM dried out"? Tell me what you find. If this were a wide-spread problem, why do we have so many 10-year-old PCs showing up at refurb outlets? Do you really think that those places de-lid, and re-paste, old CPUs, that sell for $10? It woudn't be worth the labor, at least, not in the USA.
But hey, n00bs seem to know everything, so, carry on.
https://www.arctic.ac/us_en/mx-4.html
Realistically, how long are you planning on using this PC? For more than 8 years? If so, then perhaps the TIM is a consideration. But for most people, it's not.
Edit: Sorry, perhaps I was a little harsh, above. It was intended with a dose of sarcasm.
The point that I was making, was that, possibly, yes, processor lifespan might be shorter, with TIM under the heatspreader, rather than solder. But how much shorter are we talking about? Intel retail-boxed CPUs are warrantied for spec. operation, over spec. temp ranges and voltages, for 3 years.
In my experience, they will easily go 8-10 years, or longer. Bad CPUs are rare.
Now, in my somewhat extensive experience servicing SOHO PCs, some of the AMD dual-core 90nm and 65nm CPUs, that had TIM, DID have un-explained temp rises, that was NOT cured by re-pasting (on outside of heatspreader) and a new heatpipe heatsink. So yeah, it did seem like their TIM was drying up, after 5-6 years.
But I've really only seen this on AMD CPUs, and not Intel, for the most part. Remember, those AMD CPUs were 125W dual-cores, and slightly "factory overclocked" at stock speeds. (AMD X2 5600+ and X2 6000+ OEM PCs, I'm looking at you.)
So, IMHO, this really only is a factor, and not even that much of a factor, if:
1) You intend to keep the PC for more than 5-6, maybe 8 years, and
2) You intend to overclock it.
In that case, get a soldered Ryzen CPU, by all means.
Btw, I'm not even totally sure, with the lower cost, Ryzen APUs may be pasted, perhaps someone can verify.
Edit: heatspreader TIM dry-out, is an issue, kind of like SSD wear-out. Yes, both are technically true, but in a practical, day-to-day sense, they don't matter all that much, unless you are subjecting your PC to extremes.