Thanks for the photo. Most plumbers probably did not know these existed, including myself until a bit of research. Most seem not to care, so let me try and explain my point. [The "YOU don't get it" was directed to a DIY'r not the plumbing industry]
As you mentioned, a expansion tank will be stressed at its max at 149PSI, and very likely an older one will break the bladder.
Then the last resort is the T&P valve, which as we all have seen on videos, the failure thereof cause a missile launch, unless your supply lines hopefully and likely broke first. More than a few homeowners have fixed a leaky T&P with a pipe plug....
Since maximum pressure safety and water line breaks have been such a subject of threads here, it seems adding a mid level pressure release is a very cost effective move.
If you have a expansion tank, adding a simple 8$ adjustable relief valve [not the combo shown in your photo] and setting it at 100 PSI, means that the tank takes pressure increases from say 50 to 100 without opening any valves.
Then if you get to 100, a few tablespoons of water out the valve prevents over stretching the bladder. When that valve fails, the last line of defense is the T&P and your old toilet supply line. If they hold, you might make the evening news.
Since I have given up on complaining about expansion tanks, I simply propose that a mid-level relief valve preserves their integrity, and makes the 150PSI T&P a true last resort.
And I do have a few older T&P valves set at 125PSI, a much more reasonable number.
The safety Guru over in Electrical would love my third level of protection!