Bruce Lee is considered the Father of MMA because he advocated a number things that would later make up his JKD. Some of these things were borrowed by MMA practitioners.
As Tez pointed out, there are some flaws in this idea. And I don't think it's accurate to say that MMA borrowed these ideas. Nobody had a monopoly on them. Lee definitely swam against the tide, but he certainly wasn't the only one.
Bruce advocated - Fighting in all ranges.
As has been brought up before, Greek pankration (as just one example) was doing kicking, punching, and grappling long, long before Lee started talking about it. Champions were crosstrained in wrestling and boxing.
Many systems years ago were specialists in 1 or 2 ranges. Some Styles were great at long range with kicks, Some were good at the punching or mid range, some at close range and others excelled at ground work. Some were good at punching and kicking, some at punching and close range and some at close range or grappling range.
And many at least paid lip service to most of the above.
MMA has looked to kicking arts like Muy Thai for long range techniques, boxing for punching range, originally Wing Chun for the trapping or close range (but this has largely been dropped) and BJJ for the grappling range. No on prior to Bruce was doing this far as I know. Then again maybe someone was already head of the game but didn't have Bruce's notoriety.
MMA has never, in my view, looked to wing chun for trapping. There have been one or two wing chun-trained fighters in MMA events. Particularly early on. But, having only seen their appearances in early UFC, they didn't make a favourable impression. Interpret that however you will.
Bruce advocated - Making trainings alive and emphasized sparring so that practitioners would learn to apply their techniques in a more realistic fashion against a non cooperative opponent. In this way you can try your art under pressure, learn and work on distance & timing, etc.
Boxers, wrestlers, savateurs, fencers... were already doing that.
Bruce advocated - absorbing what is useful and discarding what is useless. This however is largely misunderstood. Many think this mean take any technique and combine it how you like and if you like it and if you can make it work for you then great. This however will not make what you do JKD. It might make it MMA but not JKD. First how do you determine what to absorb and what to discard? That's the missing key. Yes Bruce researched many styles but he didn't take just anything. He researched many arts and ran them through a filter, to remove what was useless. That filter is 4 main principles of JKD which are Simple Direct Economical & Non Telegraphic. If you what you abosrb from other systems does not follow these 4 principle as a guide it is not JKD.
This I mostly agree with. You did say "if you can make it work... " And I think that's the built-in filter of MMA. If you can make it work in a bout, then good on you. That doesn't necessarily mean it will become a standard part of any "standard MMA curriculum." We've seen fighters like Cung Le and George St. Pierre pull off spinning back kicks in the ring lately. But I don't see those catching on as a standard part of MMA training.
I do think that something needs to fit within a conceptual framework to be called JKD. It's not enough to simply say "I've absorbed this from my studies of X, therefore it's part of my JKD." That said, the principles and concepts are broad reaching and flexible enough that there's still a fair amount of room for debate about how those concepts are applied in making determinations.