At the front I'm using the original ones. For the back, I changed them to the same as the front ones. They are quite good - compared to accessory speakers, I really was not satisfied with anything ≤100€/pair. I checked the original ones and the bandwidth is really good. The only thing I would complain about is the position of the tweeter (as it is the same as the mid-range).
I added an Audi trunk subwoofer and I'm really, really satisfied with the sound. In my Twingo I'm using an OPTION Drive12RA (it is small enough to fit into a 14" spare wheel
see here - 100% purchase recommendation by the way).
The problem with the original rear speakers is the foam surrounds - when they age, they tear a lot. But the front speakers don't have foam, they have rubber surrounds. They age too, but it is almost not measurable in 30–40 years. So you are good to go with them.
I think the biggest problem in the original setup is the radio/amplifier - you need a usable equalizer, as the original speakers are dominant in the mid-range (what can sound bad). A bit more to the high range (>8 kHz) and less in the mid-range (800–2000 Hz) on the equalizer and it brings big difference.
It is a myth that audio stuff ages badly - most of the equipment doesn't age at all, at least you can not hear this. And there is no new technique, almost everything is marketing. There are a few really big improvements, but everything is inside the preamp / amplifier (e.g. digital equalizers/filters, Class D high-efficiency amps). But speakers are the same as they were 50 years ago, with no real improvements. And if you have paper or rubber surrounds… everything is fine.
And one more tip: a bit less bass to the front/rear speakers and more to a subwoofer will also improve the sound quality of these speakers (no matter if original or aftermarket), as they have to reproduce mid sounds and bass from the same membrane, which can sound distorted. The less bass you send to these speakers, the clearer the sound will be.
And to be honest, I'm really not easy to satisfy, when it comes to HiFi. Even, if I lower my expectations in Car-HiFi a lot - because it is a catastrophic surrounding and extremely hard to clean up the sound. Just to say, my expactation isn't "at least I recognize the song".