Sadly, my MIL was conned into American Funds by a door-to-door Edward Jones salesman. I had to show her the numbers of what the front-load and expense ratio would do to her over time. I'm not sure of the exact fund she was invested in, but it had a 5.75% front load fee and around 0.7% expense ratio.
She is around 60 years old, so not really looking at a 40+ year time horizon, but a 40 year timeline could look like this.
Investor A
Invests $500 per month in Vanguard index funds at 0.05% expense ratio, gets 7% return for 40 years
Investor B
Invests $471.25 per month ($500 minus front load fee) with American funds, gets 6.35% return (0.7-.05) for 40 years
Investor A ends up with $1.257 million, and Investor B ends up with $996,000 over the 40 year period.