Exposure to or investment in the stock or bond markets is one method of accumulate funds for retirement, but mutual funds are not necessarily the best vehicle to achieve that goal. With the advent of Exchange Traded Funds (ETF's) with their low costs, a very viable alternative to investing has been created. Most ETF's are cost competitive with mutual funds and allow investors to have more control over their capital than do mutual funds. Here's why: mutual funds must keep cash on hand for redemptions which means they can never be fully invested. That will create a drag on performance that ETF's are not faced with. To compensate for that drag, funds must use strategies that you haven't considered or may deem risky. Not so with ETF's.
Diversification is a great concept, but the problem investors (and a great many advisors) have is what to diversify into and when. Diversifying into large growth stocks in 2008 was not the best investment strategy for long term investors. Missing the futures boom through the 1980's-1990's wasn't a good investment approach either. And mutual funds can't get you into the futures market efficiently. You may not have been in these markets, but there is a lot of evidence that 20% of a portfolio should be in the futures market most of the time.
There are additional assets such as real estate and venture capital that should be a part of an investment portfolio, whether in a tax advantaged retirement account or a taxable account growing to provide funds for retirement.
A lot of other factors impact when, where and how to invest for retirement. It would be impossible to discuss them all here. But I hope I've given you some basics to investigate.