Pretty much this. I can't afford to commit that level of income to investments, and even if I could, the impact on my financial standing would not be significant (probably? I have no idea what kind of % return you get on an index fund)
Rule of thumb is often an average 10% growth for index fund return (e.g. S&P 500) - this would be over many decades. If you can't put in 10%, at least put in up to what an employer matches - unless you are very low income, it's worth it and once it's sequestered from your day to day budget you won't feel the difference of 1-5%.