What if we have a retro-like sentai ?

They do so today too. Both ToQ 1Gou und ToQ 4Gous actors practice martial arts and their is one episode of ToQGer were they show off their skills.

Actors today rarely do their stunts themself anymore not because they cant do it but because they arent allowed to do them anymore.

Lots of actors do practice martial arts.
As for their own stunts, many can't risk injury while filming.
Many actors are just pretty faces, yes, but to compare the two is rather unfair as stunts have gotten noticeably more complex.

As in a retro theme/motif or a Sentai that just acts like it was made in the 80's?

Not only 80's but 90's too.I would say a homage.

7L, you ask this question a lot, lol.

They do homages to older shows all the time, but Toei has a system that they like now; a system that is profitable. Sentai is like their fourth grossing property (I think...don't quote me on that) and it is making enough money to pay the bills and keep it going.

They tried a more simple sentai; one with less gadgets, a darker story to attract the whole family, and a smaller team and less supporting characters so each person could get the proper amount of focus, and it is considered a huge failure. I am of course talking about Go-Busters which came after the biggest season they have done to date with the most tributes they could jam pack into 51 episodes and six movies. Go-Busters was simple and it failed.

The pendulum has swung towards a more gimmicky time where the target audience (the people that want all the toys) want something to collect. Look at Yokai Watch, that show is all about collecting it is blowing every other Toei/Bandai property clear out of the water.

It is more about making money now; we just have to accept that shows like Liveman, Jetman, and other in that realm are a thing of the past. I hope that style of show makes a comeback though, but I honestly don't see it happening anytime soon.

There are some good sentais today that had great ratings

Who is the best animal ally?
Navi

If someone wants an actor doing his own stunts, watch Goseiger. The stunts in the final few episodes were mostly done by Alata's actor himself.

If you only count those that actually look like animals- the buddyroids.
If robots like count, as Navi is somehow robot-like: Datas from Goseiger.

Doggie

Oh yeah, Doggie wins this hands down. Animal ally, mentor, extra ranger, and a badass swordsman. No contest.