SERA requires flight plans for all controlled aerodromes

I actually think, despite the oddities sometimes, Germany has the best solution for it, as I have done and heard many people close the flight plans before landing

It has gotten a lot stricter lately. Nowadays they will only let you do that when you are basically in the pattern ("landing is ensured") which means you have to stay on their frequency for much longer than you should. While Germany does allow in air closing of flight plans, it is something ATC don't like and I would not be surprised if in the future it gets banned altogether. My theory is that they try to make it easy so they don't have to chase you down afterwards when you forget to close it.

My reading of SERA.4020 is that Germany will have to give up in air closing. Here's what it says:

(a) An arrival report shall be made in person, by radiotelephony, via data link or by other means as prescribed by the competent authority at the earliest possible moment after landing, to the appropriate air traffic services unit at the arrival aerodrome, by any flight for which a flight plan has been submitted covering the entire flight or the remaining portion of a flight to the destination aerodrome.

...

(d) When communication facilities at the arrival aerodrome or operating site are known to be inadequate and alternate arrangements for the handling of arrival reports on the ground are not available, the following action shall be taken. Immediately prior to landing the aircraft shall, if practicable, transmit to the appropriate air traffic services unit, a message comparable to an arrival report, where such a report is required. Normally, this transmission shall be made to the aeronautical station serving the air traffic services unit in charge of the flight information region in which the aircraft is operated.

I usually put all AIS phone numbers I can find into my mobile phone when I go on a trip. The last time I had a problem was Poland where nobody at AIS picked up the phone. Well, I've tried what I could.

I find this rule incredibly stupid and incredibly hard to believe. Please send me the AIP reference and/or appropriate legislation and I may start to believe you.

Not that I would have a problem with you not believing me...

AIP Germany ENR 1.10:

4.5 If special circumstances unknown prior to take-off make it necessary, the pilot may file a flight plan during flight. The air traffic control service will receive the flight plan data on the designated radio frequencies and forward them to the AIS-C.

This does not apply to continuing flights after intermediate landings and to flights abroad.

If you request in air filing, it has to be due to "special circumstances" that you did not know. This includes negligence, you have to know what you were supposed to know and what you could have known if you were a responsible pilot. The relevant portions of air law dealing with flight preparation are very broad (you have to familiarize yourself with all information required for the orderly conduct of the flight). Air filing for flights abroad are generally excluded even in case of "special circumstances".

As I mentioned earlier, usually nothing happens and ATC will do it but the threshold is high and you should not expect that you can do it. If you lose time because you had to avoid thunderstorms and need a night VFR plan, that should be OK. If the weather is different from what was forecast, that should be OK. But the "let's go VFR and see what it will be like, we can always file IFR enroute" attitude you find in the US is unfortunately not possible in Germany right now.