Investing in a reputable big casino makes sense if:
1. You have at least 1 BTC. Less than that, transaction costs and the fact you may need it means it's not investable. Investment is long term, not short term.
2. Invest what if you did lose you might feel alot of pain but you'd still be solvent. One note: I think the risk of a big dice site running with your coins is extremely minuscule. Seriously, they are risking death doing that.
3. I don't see many big whales anymore. They appear to have gone extinct. So the variance is lower, meaning lower variance.
4. BE CAREFUL NOT TO BE TEMPTED TO DIVEST ON THE SITE SO YOU CAN GAMBLE IT. This is a common pitfall. Some sites allow you to make an account permanently unwagerable. But most for reasons beneficial to them, don't restrict this. If you cannot fight off the occasional urge to use that investment bankroll to gamble, then don't invest.All that will do is give you easier access to flipping coins and losing your principle.
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This may be the golden age of investing. If BTC is say, $50000 in 5 years, you're doing well right now.Most sites you are gonna get 50-100% annual returns. A Bank CD pays 1% on a shit fiat currency that loses 2-3% a year.
You definitely don't need at least 1 BTC. You can make a profit with any amount, and lots of people don't have an entire Bitcoin.
The risk of a dice site pulling an exit scam and running with everyone's coins is higher than you make it seem to be.
I see whales all the time, you must be on the wrong sites.
#4 is the only one I can agree with you on, I also see it a lot.
Ok yes you can have .000001BTC. Whatever is alot of money to you that's your threshold. It's a waste of time for anyone even a homeless person at some point
When was the last time a site that invested significant time marketing and building a quality product jumped ship with investors money? I'd like to know if you can answer that.
I haven't seen whales in a long time. They only seemed to appear on bitdice.me which I don't frequent anymore. A whale is someone firing down 5 BTC+ a roll or more.
You never know when you get the gambling itch. Don't do it or you are better off just keeping your coins in a paper wallet.
Anyway, I agree that investing to online casinos to add to the house bankroll is really risky as well but if you just invest regularly, you will be like the house to earn and gain profits at the end of the day even you have negative profits at any point because you have with you the power of the house edge advantage. 
