A lot of car makers left F1 in the past, Mercedes has returned, but Honda, BMW and Toyota have left.
Only because they were having their ass handed to them on a plate. Toyota achieved literally nothing in their F1 stint, BMW did get some wins, but weren't competitive enough to justify the investment. Honda ditto, but left at the wrong time (the post-Honda Brawn team won the 2009 championship with the Honda designed car.)
And there are other racing series, which may be more road relevent. The Audi R18 e-tron has a Diesel hybrid drivetrainm with flywheel based energy storage. Very road relavent and innovative in the field.
Motor Racing does help drive innovation but in a sport where the FIA have virtually done away with any concept of innovation, it'll be difficult to see how this new formula will enhance the sport or spur innovation in day to day cars. Fans are leaving, sponsors are worried and that means no money and a dead series coming soon.
It's not all about innovation. It's also about the grunt work of refining what you have. That's why Mercedes are dominating even the other identically powered cars. They've done the best job within the rules defined.
And there are lots of ways to innovate in chassis and aerodynamic design. The current crop of F1 cars have a very diverse array of front end designs.
And lets be honest, most F1 innovations don't translate to road cars anyway. The biggest influence of F1 and other motor racing has been in the engine management and fuel injection areas. Racing aerodynamics? Moot. Suspension design? Not applicable to most road cars. Sequential gearboxes? Came from bikes anyway. Tires? Irrelevent unless you only want your tires to last a week.