My original mat is about 25yrs old and is an Olfa.
Definitely worn and I knew it was time to replace.
I still have it and it is on the corner of the table that I don't use so much.
So a couple of years ago, I bought a new set of 3 .... you are correct, the new mats are not as thick.
To me they are more slippery when cutting and while they are still considered "self-healing",
because they are not so thick for the blade to cut into is probably why the slippery feel.
I still don't really like the new mats, but not much I can do about that.
Whenever I ask about it in shops, I just get a blank look or stare, or shrug.
Often have been given the comment, that nothing is like it used to be. IOW get over it!
My new ones are three 24"x36" mats. My table is 48"x72".
The four sit still and I never used the clips that came with them, nor see the need for duct tape.