Continuing to work on HL1 mods from a personal perspective isn't a waste of time. I still map for HL1. I've tried Source and have a lot of beef with the SDK which has put me off so far, nevertheless I will make the switch when it suits me. Mainly I've been mapping on HL1 for ~6 years and want to put out a release I'm proud of.
Will my maps get played? Well probably not much, but then people on these forums have equally poured months of effort into quality CS and CS:S custom maps which no-one will play either. I guess like these people I map out of enjoyment and love of the game, regardless of if it'll get playtime.
I do agree however that a mod like SC that has huge potential on Source did miss the boat a bit by not getting a Source release out sooner. I never played that mod, though I heard it was fun it did look like shite even by HL mod standards, so yeah I'm looking forward to seeing a pretty version on Source. 