Though the Mariners' history is not the most successful in terms of postseason play, there are several older franchises (like the Rangers, Padres, and Brewers) that have never won a World Series as well. Should they be penalized more for existing longer without a title to show for it?
When the Mariners did make the playoffs those four times, they made the ALCS on three separate occasions, which is still respectable for a franchise that barely makes the postseason at all. One incredible feat for the Mariners is their seemingly unbeatable record of 116-46 in 2001; the most regular season wins in all of MLB history.
For a purportedly bad franchise, that is an amazing record to have and one that will certainly hold for a long time.