One issue that I did see mentioned above was the type of surfaces you walk on has a direct effect on tread life. For example road walking is like using a "cheese-grater" on your soles. If the soles have a composition to provide additional cushioning, the compound may be too soft.
I am 5'11" and weigh (too much). On all three Caminos I wore Keen Targhee II mid-high boots. The tread last about 1,000 before the fiber mid soles pop though in the heels. After the first two Caminos (Frances) I had the boots resoled professionally using factory "findings" (parts). This is about 1/3 cheaper than new boots. The advantage is that the uppers are perfectly worn in and suited to my feet.
I use prescription orthotics from my podiatrist as I "pronate." My feet roll and outside edge heel wear is prevalent. The orthotics compensate for that geometric flaw in my feet.
So, no boot last forever. But your wear rate suggest something else at play. My guess is either you have a step that "drags" your heel causing excess wear, you are walking mostly on paved surfaces, or you are buying boots with too soft some composition.
As regards replacing your boots along the way, as others have pointed out, there are many small shoe shops in the larger towns and cities. There is also a Europe-wide sports superstore called Decathlon. They have at least one full aisle devoted to hiking boots, at least in every store I have seen. In fact, some stores have one aisle for women and another for men.
Along the
Camino Frances, there are Decathlon stores in the outskirts of: Pamplona, Logrono, Burgos, Ponferrada, and Santiago, among many other locations across Spain. Check out the web site at
www.decathlon.es. Drill down on footwear for trekking or mountaineering. You will see many models in their house brand "Quechua." However, they also carry some "world brands," such as Merrill. Use the search bar at the top of the page to search for brands.
Here is a direct link to the Merrell boot page for men:
http://www.decathlon.es/C-310002-merrell
Here is a direct link to mountain boots for men:
http://www.decathlon.es/C-1197569-botas-de-montana-hombre
If you are wearing the same brand and model, you need only your style and size to replace the boots with the identical model.
The Decathlon house brand offers excellent value for money. However, you must try them on with socks in the store and ensure they fit comfortably before you walk out in them.
I hope this helps.