Can you  post a screenshot of your batch task condition?

What I'm trying to do is deleting all the ones older than 2016. There's no error message and they're really removed from the TM after each go .... But after having applied this script about 10 times, I'm still in the year 2009 ...

Looks perfect, so I don't understand why it doesn't work. I use batch conditions for search and replace.

But if you want to have this operation simpler, use the same filter on a TMX export and then create a new TM from that export.

Again, try to use TMX export for this. Will be certainly faster and you can use same kind of filter.

That sounds like a good plan. What programme would I be using to edit the TMX export? Any XML editor?

You export the TMX directly from Studio. To do this, open the TM in the TM View in Studio, create the necessary filter and save the filter. Now close the TM, right click its name in the list on the left and chose export. Set the filter for exporting segments. After the export has finished, you should see how many segments have been exported. You can compare this number with the total in the current Tm, it should be significantly higher.

Now create a new TM, using the old one for "Create from", this way you will have the settings in your new TM exactly the same as in the old one. Import the TMX there and you should be done. There should be no need to edit the TMX. Should you however like to edit TMX files, you can do that directly in Studio too, there is a special file type for that on the app store.

Thank you Jerzy - brilliant idea and it worked like a charm! I'm now down to 3,000 TUs in the new TM and all the outdated ones (more than 70,000 in the end!) are gone - which is exactly what I wanted. 

Glad I could help. Stay safe & all the best.