If you know your services are going to be used to commit a crime, then yes, that makes you an accessory and basically all jurisdictions (I know basically nothing about French criminal law) can prosecute you for that. Crime is, y'know, illegal.
Why aren't all gun manufacturers in jail then? They must know a percentage of their products are going to be used to commit crimes. A much larger percentage than those using Telegram to commit one.
The usual metaphor is child pornography, but let's pick something less outrageous: espionage. If a spy uses your messaging platform to share their secrets without being detected & prevented, that's using the service to commit a crime. Now, if you're making a profit from said service, that doesn't necessarily make you a criminal, but if you start saying "if spies used this platform, they'd never be stopped or even detected", that could get you in to some serious trouble. If you send a sales team to the KGB to encourage them to use the platform, even more so.
The second amendment gives weapon makers some extra protection in the US, but they do have to be very careful about what they do and do not do in order to avoid going to jail.
> They must know a percentage of their products are going to be used to commit crimes. A much larger percentage than those using Telegram to commit one.
Do you have the stats on that? I don't, but I'm curious. While I don't doubt the vast majority of people using Telegram aren't committing a crime, I know that the vast majority of people using guns also aren't committing a crime.
That's because you're assuming facts not in evidence and painting the broadest possible argument. Obviously we don't know the details yet, but it's not unlikely that this situation was a bit more specific.
Consider:
F: "We want you to give us the chat logs of this terrorist"
T: "OK!"
F: "Now we need you to give us the logs from this CSAM ring"
T: "No! That's a violation of their free speech rights!"
However, I still argue that wanting to provide secret communication (which Telegram actually doesn't do) is not abetting crime or helping it more than any other product.
In fact, in my humble opinion, it's the opposite: Private communications are a countermeasure against the natural tendency of governments to become tyrannical, and thus maintaining one is an act of heroism.
That's an easy enough statement in the abstract, but again it doesn't speak to the case of "Durov knowingly hid child porn consumers from law enforcement", which seems likely to be the actual crime. If you want to be the hero in your story, you need to not insert yourself into the plot.