When seat belts were first introduced, did people complain that putting them in every vehicle would be too expensive, that they could easily be bypassed, etc?

Yes they did.  There was all sorts of whining and bellyaching from the auto manufacturers - at least the American ones.  For years that was the pattern - the Feds would impose some new safety or emission rule, and the Big Three (GM, Ford, Chrysler) would claim that it was too expensive, was technologically impossible, it would take many more years to develop than the law allowed, etc.  Then the foreign automobiles would meet the requirement and the Big Three had no choice but to get in line. 

This was not a new pattern -back in the early days, when autos were rolling death traps, the manufacurers fought againts having to use safety glass. 

Not in Oz. We grumbled a bit because we didn't think they were necessary but they've proved their worth.

Not long after they were introduced I was pulled over by a cop and booked for not wearing mine. I knew I was at fault so I didn't complain and he said, "You're not arguing about thisl, sir."

"The FA Cup Final starts in 20 minutes. If I stand here arguing witnh you I'm gonna miss the start."

"Since you're taking it so well can I suggest that you don't pay the fine before next Wednesday."

"Why?"
"I heard on the radio that the world is going to end on Tuesday."

Any cop who can make me laugh while he's booking me isn't doing a bad job. : )

Well not too many people wore them until seatbelt laws took effect