I was reading this thread to understand how good Laudo models are, so sorry if I contradict you.
The 131 successor in rallying was Lancia 037, made always by Abarth.
(and also in road cars, Ritmo was 128 successor, while 131 was a larger car).
Fiat group chose the Lancia brand, but the car development didn't start specifically as a Lancia. It could have been also a Fiat.
(Even if it was "cousin" of Beta Montecarlo turbo).
And yes, 4WD introduced by Audi made racing difficult for 037, but it was anyway competitive (we remember it for the last 2WD championship win in wrc - 037 has been able to win costructor championship in 1983, between 1982 and 1984 won by Audi).
Fiat Ritmo was just a smaller car, front wheel drive. I think also different racing group from 131, 037, Audi Quattro and so on.
For example Bettega and Cunico raced with Ritmo.

Fiat Ritmo wasn't at all a successor of 131.This time the 131 sits alongside a successor, if in chronological terms rather than motorsport impact.
Fiat did try the Ritmo in rallying (and circuit racing), but the arrival of the Audi quattro re-wrote the rules for what was required.
The 131 successor in rallying was Lancia 037, made always by Abarth.
(and also in road cars, Ritmo was 128 successor, while 131 was a larger car).
Fiat group chose the Lancia brand, but the car development didn't start specifically as a Lancia. It could have been also a Fiat.
(Even if it was "cousin" of Beta Montecarlo turbo).
And yes, 4WD introduced by Audi made racing difficult for 037, but it was anyway competitive (we remember it for the last 2WD championship win in wrc - 037 has been able to win costructor championship in 1983, between 1982 and 1984 won by Audi).
Fiat Ritmo was just a smaller car, front wheel drive. I think also different racing group from 131, 037, Audi Quattro and so on.
For example Bettega and Cunico raced with Ritmo.