Thomas Edison didn't continually promise investors he would have...

Thomas Edison didn't continually promise investors he would have it finished imminently for many years. He didn't say various bugs were fixed when they weren't. He didn't say he had a fully functional prototype then years later fail to get any more working and for some reason never show anyone the prototype actually functioning despite saying his plan was to rent out the prototype at a fee which was going to help fund production and development of future light bulbs. I'm quite sure that if when his promised production line light bulbs were not working years after he claimed he had a working model which was going to be rented out to fund the project, people would have been highly dubious about his ability to actually produce functional light bulbs, but in Edison's case, he didn't claim to have a functional light bulb until he actually did.

Nikola Tesla actually invented the light bulb but he never got it to function well enough to be of any commercial use. Edison took the invention and refined the technology to make it practically work and become a product which could be produced and sold for profit, something the original inventor never managed. The fact that Tesla invented the concept and was the first mover but never made any money out because he was unable to solve the technical challenges required to make it work practically (for memory the trick was using a rarified noble gas to prevent rapid corrosion of the filament and a sufficiently robust filament which ended up being tungsten), speaks volumes.

Sure, lack of success to date doesn't guarantee failure, but lack of success to date does not guarantee success! Looking at another light source, the LED, many companies poured huge resources into their development and failed, and only the one which succeeded took the prize, with the others wasting all their time and money. Huge dedication of time, money and expertise do not guarantee success. Success is not inevitable, and when someone has already claimed success but years later can be seen to clearly never have achieved success, scepticism about future success is highly appropriate, and it is completely reasonable to assume that if success does eventually come, it will likely take a long time and imminent success is a very dubious assumption.