Considering the supply vs global population = 0.003BTC per person to go around.
Then other factors like not all 21 million will be in circulation due to lost wallets/deceased hodlers who never informed anyone where their seed is, and also an increasing global population and future generations who all want in.
Do you think 0.1BTC will be like owning a mansion?
No. If we assume bitcoin will hit $1 million (which is pretty generous), then 0.1 BTC will be only $100,000. If bitcoin hits $5 million, then it will be worth $500,000. With potentially very high inflation in future years, this may not be as great as it sounds today. However, owning any kind of bitcoin is probably better than holding cash.
How many people have $100,000 or $500,000$ today ? This may not be a good amount of money if you are living in a developed country but in many underdeveloped countries, a person possessing this much amount of money will be considered a millionaire.
If you have 0.1 bitcoin, just hold it in a hardware wallet and see what developed after 10 -15 years. It could be a life-changing investment.
In underdeveloped countries that amount is considered as fortune, you can have a decent house and cars with business on your own, if you can hold your asset and you believe that the future will be nicer, considering holding and allow the progress to take place, in that process you will generate a decent amount of profits and who knows what would be there awaiting this crypto currency.