I just want PR have better plots again.

It helps that Power Rangers didn't come entirely out of nowhere- it was right on the heels of TMNT. The two were close enough in theme and tone that it was a smooth transition from one to the other.

Yes, and to add to what you said, I was also thinking maybe the setup somewhat resembling shows like Saved By The Bell and to an extent Archie Comics also helped as well maybe?

Can't imagine it hurt it. "Oh it's live action like a sitcom but it has action like a cartoon!" wasn't a new concept at that point but it was still a novel one. I think Godzilla was in the process of or had just had a resurgence in popularity in the US and abroad (from Japan's perspective) as well?

Did Saban sell Power Rangers to Disney because they (he?) needed the money?

I heard that Fox family channel debt was huge and Disney purchase by 3 billions.

Saban didn't sell Power Rangers specifically. Disney bought out Fox Family, which included Power Rangers.

Godzilla did have a resurgence in popularity in the US before the 1998 remake. I'm not sure if that helps to the success of Power Rangers or not.

Saban was actually taken back on the purchase if I recall correctly. Apparently Saban Entertainment brand was a subdivision of the Fox Family Company, a former asset of the now defunct 20th Century Fox Film Corporation, and when Fox sold it to Disney, that included the Fox Family Network (ABC Family, now Freeform), Fox Kids, and Saban Entertainment, among other assets. Basically Saban had his company ripped from under him and sold to Disney. That's why in the 2000's we saw more independent production companies that signed exclusive deals with studios instead of starting up within a studio to prevent such a business transaction from occurring, at least until the owners decide to sell or go public.

Should in sentai have more realistic mechas ?