It's all about personal decision my friend, holding bitcoin is very easy, I don't have to separate funds or bitcoin to different wallet to hold my bitcoin if I really want to hold, if I don't have any personal and important need for the money, I hold it, even In one wallet, I leave it there as an investment.
And on the other hand, just incase something important comes that requires I must sell some bitcoin to raise money to sort it out, i myself know what amount of bitcoin I've kept as an investment and whatever expenses I am doing, I make sure i never spend to the extent of entering my investment, lets say for example, my investment 1 bitcoin, and I have 1.5 bitcoin In my wallet, I know what ever transactions I am doing, it must be with that 0.5 bitcoin, the other 1 bitcoin will never be touched for obvious reasons.
Though what you say is right that one should have a clear count about how much of their assets they are willing to hold and they shouldn't spend that even if the wallet for their general expenses and holdings is the same, a lot of people tend to use separate wallets for both purposes so that they don't need to open and use the wallet they are using to hold their Bitcoins or other cryptocurrencies and they only open and make transactions with the other wallet.
But just because a lot of people do that it doesn't necessarily mean everyone should do the same thing, everyone has their personal preferences as you said, so some might have no issues keeping all their funds in one place while others might find it easy to separate their wallets for different purposes.