This only counts Lucas's influence on SF. What's bigger is his influence on the film industry in general...
-Fewer big-budget blockbusters, of whatever genre.
-Without the Star Wars films, a number of visual effects wouldn't be available. Film and TV effects would be a lot less sophisticated.
-No THX sound system.
-No Pixar, either on its own or as a studio.
Also, there would be one fewer collector of Maxfield Parrish paintings or Tucker cars. And someone else would get to say "That isn't fair!" to Eddie Murphy in that movie...
Orville is right. Lucas influence has little to do with Sci Fi. Furthermore Star Wars IS NOT SCI FI. It is Fantasy.
Lucas missing would have less effect on either genre than on the industry:
For starters:
No THX sound system - this means surround sound comes much much later.
No merchandising and franchising of movies into action figures, t-shirts, roller coasters, comics etc. This is Lucas big contribution to the movie business. Sure someone else could come around but once again that is later.
All of this could butterfly the coming of the big blockbusters later on. High concept, High special effects, easy to sell movies. Maybe not entirely but if you take number 1 and number 2 off how can something like Transformers 2 happen? Studios would simply not see the potential. Lucas made them see that potential
No mainstreaming of geek and fandom culture. Star Wars helped bring geeks into the light in the 1980s and still does today. This in turn heavily affects video gaming and comic culture and mainstreaming.
The list goes on and on.
Including
No Pixar
Less development of Special Effects
Less development of Digital Video
And you do not even need to kill Lucas. He wanted to be a race car driver before he went to college. Just have him fufill his childhood dream.