What about the hodler who has a portfolio of lets say for example 25 different cryptos/altcoins spread between dozens of centralized exchanges.
there aren't really "dozens" of legit exchanges to use and most traders only use 2 exchanges tops. and if you are using an exchange then you can't call yourself "holder", you are a trader and you need to act accordingly meaning you are not "storing" coins on exchanges you are keeping them active meaning you buy and sell these coins constantly. and if you are doing otherwise then there is something seriously wrong with your strategy.
additionally i would like to point out that holding 25 altcoins is a terrible decision because they are all moving down with bitcoin and rarely move back up with it.
How does this person withdraw all of the 25 cryptos balances for the sake of proof of keys? Is there a easier convenient way to do this inconvenient task?
no you should not move your coins because of a silly thing such as "proof of keys" that is running on social media. you should however withdraw them if you are not trading as i explained above. exchanges are not meant for storage.
and if you are planning on holding 25 then you also have to accept the difficulties of doing so. you can't expect easy money with easy security. it is tough to stay secure.
Also does every crypto has its own wallet to accomplish the above task? If so the person has to download 25 different wallets which is another inconvenience and hassle. Is there a user friendly way?
as other user said there are multi-currency wallets although they are new and some of them have had bad bugs.
Finally what about cryptos/altcoins that due for a coin swap to the new chain or version decided by the development team? On centralised exchanges you should get a notification of this in advance by email so the person can make preparations to prevent hodling useless worthless obsolete coins. If the person downloads 25 different wallets then will he/she be notified of possible coin swaps for each wallet? Is there a user friendly way of doing this for sake of proof of keys?
all your arguments are the same. if you want to bag hold a lot of them then you also have to accept the difficulties! and if you find it tedious then you have to think about not doing it! or at least reduce the number to something more manageable.
not to mention that a coin that has the risk of becoming obsolete overnight is not worth bag holding!
Centralised exchanges seem easier and convenient otherwise can someone explain to me a easy or convenient way for what mentioned above?
yes they are easy but you are giving up safety and in a blinking of an eye you can go to zero if the exchange decides to scam you. which has happened thousands of times already.