Will I ever see Sierra's Space Quest series or any of the other Sierra Quest games on GOG? If not then I don't see much reason in continuing to support it. I like old games but if I can't get those specific games then I just don't care too much to give any more money.

Time will tell. There is no obligation for you to support gog... Nobody ever told you "BUY A GAME OR LEAVE" :o
I don't see why you're upset.

You can buy the most recent Space Quest Collection which runs fine in XP and Vista for £5 in the shops. I also got the King's Quest and Police Quest collection for £5 each. Just walked into HMV and there they were.
(I'd probably buy them again if GOG sold them!)