The Merril compression molded EVA mid sole is what breaks down fast it is also what makes it so cushy feeling when you first put them on. When I sold running shoes I would see and feel the same problem. In shoes like New Balance,Sacunoy, Nike that had cut slab thick Eva mid soles then most companies went to the compression molded EVA that solved the problem some what in running shoes. Lowa use a Polyurethane midsole that dose not break down as fast is more stable.
The outsole for the Merrill's Just don't wear well at all on trail or street. I don't know how it works if
Vibram licenses out lug design or rubber compounds? If it both Vibram may have different hardness of rubber compounds for out soles I think Merrill use a softer compound. That may explain why you get a sole that wears out faster than a harder rubber compound shoe.
New balance for example use to uses a eva mid sole with carbon rubber Infused in to the eva in the out sole they would wear out faster. Than a running shoe with a thin harder compound rubber out sole. That is why the average running shoe had life span of only 90 to 200 miles depending on the manufacture.
I have also had some bad results with out soles on Asolos and Adidas hiking boots that de-laminated because the glues soften in 90 degree weather at Joshua tree national forest. They started to flop around like a dogs tongues. Back about 15 years ago it was quite funny since I was near my car kind of cartoon like my hot dog feet were done.
Just remeber the only true thing about hiking shoes or running shoes is some kind of degradation over a period of time is built in to all shoes to keep the shoe companies in business.
So you will buy more, It up to you is the instant comfort a trade off worth the price for shoes that degrade faster or do you prefer a shoe that degrades slower but is less comfortable.
Here's some tips for buying shoe go shoe shopping at the end of the day when you feet are swollen up , Also take the socks you will be wearing , walk around that store and do al kinds of forward, backwards, stopping and starting, lateral movements to mimic trail use and wear them for at least 30 minutes minimum in the store before you buy them. Try all kinds of shoes on and to figure out what is best shoe for you before buying. Then you will be happy with your shoe purchase.
When you find a shoe that works buy a couple pairs because most companies changes something about each shoe in their line every year and it may not be the same shoe.
Happy shoe hunting,