This has been of great interest to me for quite a number of years now.
Back around the time Noah put the pintles on the ark, I found that the soles never parted from the uppers, but after some years the phenomenon started to occur even in the most expensive boots we used to use for bushwalking. I was told that it was due to the fact that the old glues were regarded as carcinogenic, hence a move to new compounds which didn't stick nearly as well. Incidentally, this may not accord with the "spin" that manufacturers may put on the issue.
Some boots/runners appear to have an inner sole made of cardboard type material, and getting these wet seems to hasten the process. But then the mega expensive runners I've tried seem to separate just as quickly.
The Camino is hardly demanding on boots from this perspective (although I wore through the soles of two pairs of new boots on my walk) and generally I wouldn't expect the treatment meted out to a newish pair to separate. But if you want boots for really tough stuff try to get those with a sewn on sole. The trade-off is that they may be heavier.
When my runners/shoes separate, I glue them together with Sikaflex 260 which generally gives me a few extra months of use. After all, if you pay 12 bucks for a set of runners ($4.99 for my most recent) you have to get your money's worth. I should add that I once bought the very most expensive model of a reputedly top brand, rated Running shoe of the year by a US running magazine, and the soles collapsed within a fortnight. Some time later I made the mistake of buying the same brand only this time it was the soles parting from the uppers in less than a week. Hence my move to el cheapos!
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Bogong