The 8% is NOT a return... it is the increase in your monthly SS if you defer benefits for a year... but not anything close to a return.
I took your chart and expanded. I started with collecting $1,000 at age 66 and I factored in a COLA for each year. I used 1%, 2% and 3% COLA.
That made a difference - the "break even point" changed by at most one year for each increase. But the cumulative amount received added up. Putting in a different amount like $24,000 to start changed the cumulative amount but did not change any of the break even years.
My life expectancy (based on the outdated tables currently being used) is 83. But most in my family live longer. My mother lived to 89 and I am better health at this age than she was. So (as long as I survive 2020), I expect to live longer.
In addition, waiting until 70 means I will have more time to do Roth conversions. That will mean lower RMD starting at age 72 and possibly lower income taxes and lower Medicare premiums.
0% COLA
Break even age
67 79
68 80
69 80
70 81
Cumulative income at age 100
66 420,000
67 440,640
68 461,894
69 483,729
70 506,102
1% COLA
Break even age
67 80
68 81
69 81
70 82
Cumulative income at age 100
66 499,923
67 521,740
68 544,042
69 566,781
70 589,898
2% COLA
Break even age
67 81
68 82
69 82
70 83
Cumulative income at age 100
66 599,934
67 622,518
68 645,414
69 668,560
70 691,881
3% COLA
Break even age
67 82
68 83
69 83
70 84
Cumulative income at age 100
66 725,545
67 748,183
68 770,914
69 793,660
70 816,337