Could we sit and study Torah without donations? The answer of course is yes. If HKB”H sees fit to allow us to learn Torah and not need money, he doesn’t need the help of the wealthy.
But that doesn’t change the fact that Hashem is using the wealthy as tools to support Torah and we owe His tools hakaras hatov for donating their hard earned money to kollelim and yeshivos instead of spending it on themselves. Even though we know that they only make money because of those learning Torah, we still owe them hakaras hatov.
We could also sit and learn without the protection of soldiers if HKBH sees fit. But right now, HKBH is using the soldiers as his tools to protect Toras Eretz Yisroel, and we owe His tools hakaras hatov for putting their lives on the line to protect the Kollelim and Yeshivos (in addition to all of the other people in the land). Even though we know that they are only successful because of those learning Torah, we still owe them hakaras hatov.