Rating decreases after win

The rating system tries to estimate your strength as good as possible. If you improve you will win against stronger opponents frequently and your rank rises.
A single game may matter for you, but it doesn’t mean that you have improved over all. Looking at 15 games gives a reasonable estimation of your improvement.

At least the rating system is not a reward system, it’s a strength estimation system.

Their rank will not drop that much. If a player times out in multiple correspondence games in succession, only the first game is rated.