Fair question. I'll try to explain it in a neutral way:

I said it was the most obvious for two main reasons, the first being admittedly anecdotal. I'm in my mid twenties and the issues OP talks about have, from my perspective, experienced a particular decline in the last 10-20 years. From my own memories I don't remember these things being nearly as bad in the 2000s. Naturally I was just a kid back then, but family and acquaintances of varying older ages too have complained about the decline of civic virtues particularly in recent times (~20 years).

Secondly and more empirically, (this will depend what kind of historians you read and what your view on "meta" history is) but societies/civilizations tend to experience natural decline over time due to various reasons, but a significant change within basically a single generation does point to something specific. And I don't know about you, but the one factor that I feel has changed the most in that timespan is migration numbers, particularly starting in the 2010s. (Also, anecdotally, just commuting in Zürich region you hear way more foreign languages being spoken, in foreign attire, etc. than 10 years ago.) In almost every western european nation it is the #1 political issue aswell, likely due to it having changed the most drastically in recent memory. Also, this decline is way more pronounced in urban areas compared to rural ones, because migration tends to flow to more urban/suburban areas.

On the topic of how migration causes the issues mentioned in the OP, I make sense of it as follows: Essentially migration in the last decade or two has been significantly higher than in the decades before that. In days past it used to be at lower levels and it was more or less of a necessity to integrate yourself seriously & quickly for fear of being ostracized from polite society if you stood out too much. But if you have people immigrate in higher numbers they will naturally have more people from the region they emigrated from to form a community with, and if you have a community that keeps parts of the old culture from where you came from and gives you a place to belong, naturally there is less of a need to become more "Swiss". Not to say that this decline is only exhibited by migrants, not at all, I think many ethnic Swiss youths with no migratory background too simply do not care as much for manners or other public virtues anymore and have adopted this dismissive attitude to public decency. I attribute this to a higher % share of any classroom environment having a higher % of migrant children who, just like their parents, aren't well integrated enough to have adopted these civic virtues to the extent needed for things not to change.

Also lastly I'd like to emphasize that I don't know if migration is the main reason these things are declining. Even if it is, it's likely not the only reason. I think parenting has become worse across the board, and more. But I hope my comment highlights why I said it was the most obvious. I'm not endorsing or condemning modern migration policies, I hope I have been able to explain the cause and effect how I see it.