IMHO to answer OP: So should we replace head gaskets as a PM? Absolutely not.
We do need to follow factory PM and check coolant level frequently while cold and flush coolant and replace heater Tees & thermostat as PM.
@mmaakk this "may" shed some light on your head gaskets. In a word "electrolysis". I presume you used Toyota Red LL 50/50 or 100% with distilled water only. But that your are not first owner, so don't know for sure what was used in the pasted. I've seen even Toyota Dealers/Lexus Dealerships put other than this or mess up on procedure. Typically evidence can be found on metal, rubber and or gaskets of electrolysis.
I've been investigating a blown engine (head gasket). This is on my 07 Unicorn w/127K which spent it's life in relatively mild climate(s) and one of the cleanest well keep rigs I've ever seen (until The Black Knight). I feel it was always very well kept because it is a Unicorn and arguably one for the rarest in the lower 48 of the USA, beings it's and 07. The up stream heater tee is blown for sure. The parts of of the tees that are in the hoses discolored to a light brown when removed and dried. IMHO what happens is as the plastic ages it starts to be saturated with coolant replacing the
plasticizer. The plastic then begins it's failure stages, it's just part of aging of rubber & plastic's.
But why did this up stream heater Tee blow so early in it's life. I've replaced Tess with near twice the mileage and 7 years older that looked better.
Three alternate reasons I can think of, somewhat supported by things I've seen working on low to high mileage rigs. Why this rig overheated other than a fluke Heater Tee going so early, but then why both looked age beyond their what would be normal for age???.
1) A fluke, where head gasket was bad or contaminate on it from factory. Not likely, as it made 10yrs & 127K miles. In this case head gasket blew before heater tee.
2) Containment in coolant system, introduce during it's life, which can lead to electrolysis (acidic condition). I've seen electrolysis in coolant systems that have eaten gaskets. Protecting gaskets is just one of the reason proper care of our coolant systems is so important. IMHO coolant is the most important fluid we must care for. Electrolysis also happens if coolant not flushed on SMG time table (old coolant). In this case I'm seen records indicate coolant was flush within proper period by Dealership. In this engine I've not yet torn down, so do not yet know if any sign of electrolysis. However I did find improper procedure (FIPG use on large O-ring) use while installing water inlet, was presumably during T-belt job. T-belt done at 7yr 78K miles. To do at 7 years gives one idea how how well this was maintained, who does that!
3) This is most likely alternate cause. Low coolant level. Just low enough that it boarder temp Sending Unit at first perhaps, running a bit hot so that coolant filled expansion tank blowing a bit out over follow until below SU. Or perhaps below temp sending unit form the start of last flush. Once below temp sending unit we get a false cool running reading on temp gauge as it doesn't read air temp well. In colder climates we get a clue when cabin heater doesn't blow hot as it should. This is because coolant is not circulating into heater core as designed. In this case the coolant was flushed at Nalley Toyota in Roswell (I've seen & had issue with this shop in the pasted). Typically this kind of work is done by a junior mechanic, getting lite service jobs as part of his schooling. After flushing the system it was not burped properly, this very common. In fact, I've never seen when coolant level did not drop after first time engine brought up to operating temp, then cooled down for 8 hours, where it didn't need more coolant added after a DIY flush job. Even when one squeezes radiator hose to get out air. Most times it take two or even three cycles. If machined flushed air may have been introduced, this has same effect.
In this case I've yet to tear down engine, but plan to. I do have evidence engine was running hot for some time. Possible running low overheating ageing the heater Tees earlier than normal. 50/50 either head(s) warped then blew combustion gases into coolant system taking out Tees or premature aged Tees from coolant system running low & hot blew then took out head gasket.
