If the platform goes bankrupt you would lose your money so it probably is wise to not let balance get too high
What about all the players who are playing MTT's regularly each day? The ones who have total buyins of $5,000+? A good number of players actually have buyins exceeding $20,000/day. Even low stakes players are playing $1,000 worth of tournaments per day.
I am 'guessing' that many of them have much more in their accounts than just a days worth of buyins.
My guess would be, if it's one of the bigger rooms, your money is fine sitting on the sites. If it's some struggling site, then maybe not.
Withdrawal wait times is often a decent indicator of a site's financial well being. (but not always).
Also, how does one hope to move up in buyin and skill level if they just take any & all money won off of the site? 10 to 15 years ago, the common thinking on this forum was, 'Keep a good chunk on there so you can grow your bankroll. Take a bit off to reward yourself for the big win.
It really depends upon what the person is aspiring to do. Personally, I leave most of it on the site (but again 'it depends'... some sites I have a bankroll on them so I can play cash games & am able to draw a bit off fairly regularly, others I'm hoping to build up to be bankrolled for tournaments).
I have no fear of 'losing it all' because I don't play at buyin levels that are too high for what I have in my bankroll.