If you want to compare the Puma with Su-76 you ahve to kill in mind how bad Puma is at max range:
Puma
Far accuracy(50) 0.25 Penetration 80
Su-76
Far accuracy(60) 0.375 Penetration 160.
That is more than x150% better accuracy and x200% better penetration at max range.
The unit is safe by staying at distance. It does need to faster than all ostheer tanks when it outranges them and when the barrage outranges Paks.
How are one supposed to counter a cheap unit that can barrage better than a mortar from ranges up to close to 90 that is faster than all vehicles?
And the pumas stats are more than made up for by being able to use it aggressively. Despite its much higher pen have you ever seen an SU76 dive for rocket arty?
The SU76 is defensive AT and support via barrage. Increasing its rotation isn't going to displace the T70 and make the new speedy SU76 meta but it might make it so someone, somewhere actually builds one. Even if only once.
As for ost not being able to dive for it, it's not exactly going to be racking up the damage with its pen. There's nothing wrong with a defensive unit excelling in the defense. It would basicly be a less cancerous Scott, assuming someone builds one and manages to vet it.
It was always meant to be a multiple glass cannon. The grenadier of tanks if you will. It can do things well and from range but if your micro slips buh bye.