As with everything, there has to be Proportionality (are the gains from mandate backdoors on everybody’s software or mandatory taps on every phone out there so great they justify the high risk of massive loss of privacy and security of the general population) and ther has to be Independent Oversight, as in, it has to happen with a Court Order which is the result of a Judge having examined the case of the police and determined that there is sufficient reason to break somebody’s privacy, same as is necessary for the Police to break into somebody’s house to conduct a search.

Given that the smart criminals - read big ones - don’t use phones they bought with an ID with contract that they pay from their bank account, and in some cases even use proprietary comms apps rather than the ones common people use (it’s not as if it’s hard to make an encrypted comms app for anybody with even just a bit of mobile development experience) things like mandated backdoors on widelly available comms apps only provide a mild improvement in Police effetiveness whilst openning a massive attack vector on millions, tens of millions, or even hundreds of millions (for the whole of the EU) of law-abiding citizens, hence are a massivelly dispropotionate solution versus other options, such as bugging the devices, direct surveillance of the suspects and so on.

Also for me personally, having lived in the UK, you cannot at all in any way or form trust the Authorities with such power as they will with absolute certainty abuse it. Also even if a country’s authorities are squeaky clean, prim and propper today (most definitelly not so in the UK, were they even had surveillance on the Green Party, but most of Europe is better), there is no guarantee the next ones will still be so (remember, most of Europe was at some point under Fascist or Communist dictatorships), and in this day an age stuff once recorder exists forever and can be later fished out and used against you even when back in the day when you said it or wrote it, it’s was totally legit.

So it’s the balance of pros and cons (i.e. the fail in Proportionality) that makes mandated backdoors on everybody’s comms apps for the purpose of surveillance, even were it would be under proper oversight by a Court of Law, be an unacceptable option.