@Konstantinos, How much does your laptop weigh? How long does it last on battery (something like the WiFi v1.3 test)?
I am not convinced that internal batteries will be too much more difficult to replace than external ones. I am concerned about their future support/compatibility/still being manufactured, internal or external. But you have mentioned that the T420 battery still is being manufactured--that's great to hear. Last I checked I had great confusion trying to make sure that a new T470 external battery would be compatible, I think there were several versions that I could not tell apart online--there were arguments about whether or not they were compatible (61+ or 61++ or something like that), so I skipped the purchase. I have had many bad experiences trying to buy batteries for old devices, so that's why I am so glad that USB-PD will at least standardize power banks up to 100W for many years to come.
Even now, as I search for "T470 battery" on Amazon I see "61++" "61" "68+" "70++" "61+" "68" "57++" and others, some without model numbers, and I am sure that some will work, and some won't, and some would come with greatly reduced capacity because they are counterfeit or old. I also see the internal 24Wh battery for sale, too, just the same.
Soldered RAM and soldered storage is bad. Internal slotted memory and storage is great, but they are just as internal and as replaceable as an internal battery. What is the problem?