It is not normal for congestion to degrade performance to the point where a network is unsable, nor is it normal for people using network software to be experts in congestion control. If bitcoin is to remain distributed and not just disappear into the cloud as a new form of centralized money, the core software needs to work correctly out of the box in the real world of ISP supplied routers.
Yes it is normal for networks to become unusable when a single service is to demanding.
"Normal" people dont open ports. By opening a port you say: "Hello Internet I am a Server now, please enjoy your stay". You should be aware of this. There might be lot of "internet" coming your way. Well, you ask for it by opening the port.
Most ISPs give you the worst possible router. If you are lucky you can just make a DMZ and forward all traffic to your actual router that knows how to handle traffic. Sometimes the hardware is fine, so you just need new software (e.g. OpenWRT), but depending on the country you life in, that might be illegal. Sometimes you want to build your own router (see fli4l) or just invest a little in a good router (a linksys wrt54gl still does amazing work).
The bitcoin core software works correctly out of the box, noone asked you to open your port and configure your router to forward traffic. While its nice that you want to support the network, it can be demanding. So -as sugested- either stop supporting (close port) or shape traffic (QoS etc., proper router, local solution)
Other peer to peer protocols and bulk data transfer programs that do file sync have dealt with these problems by automatically recognizing available bandwidth and setting limits and/or allowing the user to control them directly.
It took them years. I dont use bittorrent anymore, but I have been around when you couldnt even get your mails because a single user in your network had a bittorrent client running for over 1 hour. And that was on a 2MBit/s connection when most peopl still had dial up connections. Bittorrent used to kill routers because of the number of connections and clog up the network without proper QoS. Many modern BT clients support some form of bandwith limitation because otherwise noone would use them. In every home network I managed for the past 10 years you "felt" when someone turned on the bittorrent client. Even with QoS bittorent is hard to handle when you still want VoIP and gaming, because you can only shape the traffic in your network, not the responses from the internet.
(Examples are many bittorrent clients.) It is not realistic to expect the average bitcoiner to set up QoS features in a router supplied by their ISP which may not even have these essential features and certainly lacks decent documentation.
As I said above, most ISPs wont even give you a router that is able (by software) to do QoS. But imho it is also not realistic to expect the average bitcoiner to setup a server and open ports.
While this degradation was in effect I also observed that the stale rate seen by my miners doubled, even though they were running on a completely separate computer from Bitcoin Core.
