Galaxies wind themselves up too fast. Several years ago I wrote a galaxy collision program in which I am able to mimic collisions. When I run the program with only 1 galaxy having several hundred stars I was surprised to find that spiral arms would develop and go away only to develop again. Whether more sophisticated modeling than mine would uphold this view I don't know, but it seems to me that this might be the reason we see spirals.
Comets disintegrate too quickly.
This one is easily dismissable. On average there are 6 new comets discovered each year, comets which had never been seen before. This implies a reservoir of comets out there which is sufficient to replace the short period comets. Cometary sized bodies have been found outside of Pluto's orbit. The power of the repaired Hubble Space Telescope has been turned to the search for Kuiper belt objects.