IMO Islamic military competence didn't decrease, other militaries just developed since then.
The main explanation I've read is that Muslims are still extremely hierarchical and authority is invested in people rather than positions, which means that everyone hoards knowledge to be more powerful, everything is done from the highest ranks down, and the leaders are chosen for loyalty rather than competence, which is especially bad for modern war because of how fast-paced it is. But this sounds very much like "They suck because they're not progressive enough" which is a little
too convenient.
And here I was about to use a video editing program to splice the two together. Thanks!