Low-light city landscape photo problems

A few months ago, I was like you the OP and remembered the crappy pictures taken as what then high ISO on my old Canon point and shoot. So I tried every ISO setting on my camera, from ISO 200 to ISO 12800. As noise truly began to be seen only from ISO 6400, I first set-up the camera to never go over ISO 3200 (probably twice or 4 times as much as my old Canon).

On Christmas, I was asked to take a picture in dimly lit church (it's kind of a pleonasm I know). Since I only had the 15-45 kit zoom, I gave in and tried at ISO 6400 and ISO 12800 to have acceptable shutter speeds of a 10th and a 20th of a second. On my 27 inches screen, Capture One shows pictures of about 45 by 30 cm, which would be a sizeable print and yes, I can see noise, but I mostly see it because I know there is noise in it.

But no one will see it on a smartphone or tablet screen, one will barely notice on a computer screen, because that would be assuming the people watching the picture take the time to display it full screen and then zoom it and pixel peep, and if I were to print it, I'm pretty sure such noise will be mostly lost in the printing process.

All this was done with my X-T20, whose sensor is older, and presumably less good at climbing in ISO than the sensor of the X-T3.

Your feelings about this may obviously vary, but the bottom line is that you should let ISO going all the way to 12800 if necessary and not worry too much about quality loss.