A million in 2050 wouldn't take you nearly as far as today.
If we reduce pre-inflation return to 7%, while maintaining 2% inflation target, we get $0.685m and $1m.
The 2% inflation makes it that over 30 years your total contribution is $405k at 10k/year and $608k at 15k/year.
9% pre inflation is insane, and 7% is too. If you can do that consistenly, you will be a billionaire hedge fund manager. Ten year treasury bond yields 2.59% per year, anything above it is risky.
Sure, you can stuff 100% of your savings into S&P500, but then you will have to deal with unpredictable 20-40% drawbacks, which take years and years to correct. Last recession S&P500 basically halved.
Average annualized return from s&p over past 90 years is 9.8%. Both 9% and 7% were chosen to be below average to account for some amount of lower risk assets in the portfolio. Even with the "great recession" the peak value from 2008 was recovered by 2011 (for spx). When you retire you should have a few years worth expenses in low risk assets, which will be the source of early withdrawals in case of stock market crashes at retirement.