but rules for citizenship are all different and are being made harder and harder because, well, that's what sells today. Also some countries (cough, Germany, cough) have incredibly stupid rules where you have to give up your own citizenship in order to get a new one.
I think it's perfectly normal to expect someone to give up their old citizenship when becoming citizen in a different country. When you become a citizen, that country is responsible for getting you out of international conflicts and arranging passports (which are essentially documents that say "you should let this person in because of the good relationship our two governments have"), and in turn you're expected to turn up for the draft and decide in national policy.
There are certainly countries where gaining citizenship is a challenge, but the Dutch terms for EU migrants the minimum requirements ("speaking the language somewhat fluently, having lived there legally for five years, filling out paperwork") aren't that difficult. Getting through the process takes effort, for sure, but it's not the challenge most people in the world will face (the "living in the country legally for five years" part, mostly; without student visas or special deals between your old government and the Dutch government, you're not likely to get a work visa as any random person on earth).
It's clear to me that you don't have the experience of being an immigrant.
There is nothing "normal" in expecting to renounce to a previous citizenship if you gain another one [1]. As an Italian citizen living in Sweden I obtained the Swedish citizenship (fortunately before the current government makes it way harder) but I'd never give up my Italian one. I "feel" Italian and owe my country a lot and my happily go back at some point, but I also feel like having the Swedish citizenship is useful, and allows me to vote in a country where I have chosen to live for a good chunk of my life.
I would say that it's not just useful, it should be a basic right. In my case as an EU citizen, Swedish citizenship gives me little more than I already have (voting rights, easier to get passport), but for those who come from outside the EU, citizenship is a question that affects all their life and of their family.
Just go and ask any immigrant friend if you have any. Just stop conflating citizenship with any sort of identity or morality, there is nothing like that.
In NL the obligation to renounce is pretty arbitrary. There are a number of circumstances where it is allowed to have two nationalities, such as through marriage to a Dutch citizen.
