Go-busters was a reboot too ?

Neither are reboots. They've just played with the format/concept to inject a bit more life into it.

We could have vehicle mechas in 2018 ?

To be fair I do think Sentai needs it, it's been standard fare lately and I haven't been hooked in for Zyuohger and Ninninger outside of the mecha collecting aspect, Toqger was the last one I looked forward to each episode, so it could do with some freshness to it with how it tells its stories, it's overall aesthetic and how it presents it's characters.

Now I do like Sentai characters I do think they do their base personalities fine, but I think it could do with something a bit more different with them, for example I like the way the Ex-aid characters are presented it's a bit refreshing after Drive and Ghost.

I think I'll enjoy new shows more if I watch em in batches like I am with Ex-aid so I'll do the same with Kyuranger.

I think the lack of vehicles in Super Sentai is that they started not selling as much as they used to as time went on.
Based on personal observation Super Sentai merch's best selling stuff are mostly the robots and roleplay toys.
I'm sure the sofubi figures still sell too but from the looks of it they dont amount to much on the grand scale of things.

Could the numbers of members of Kyuuranger affect the audience and the toy sales ?

How many times has an extra Senshi been an established cast member prior to their in-suit debut?

I know Kruger was this way, but has there been anyone else?

Black Knight Hyuuga, DekaSwan, the 2nd KyoryuViolet, ToQ #7, and ZyuohBird are others that come to mind.

I'm not including the other Kyoryugers, since only the 2nd Violet was relevant.
KyoryuGold might count, but it depends on what counts as an established cast member :lol

You think they could of done a movie of each gokaiger character and their life or on their planet lasted was until it was destroyed?

As a collective i.e. Episode 0 movie maybe but individually? Nah.