7% is in line with historical growth rate (below it, actually)
Cost inflation is assumed to be 3%, which is why the withdrawal rate is 4% (4% + 3% = 7%)
Anyway a 4% withdrawal rate is fine for 30 years (the duration of the trinity study) but a lower withdrawal rate (say, 3.25%) is needed for longer timespans like the 65-ish years of life expected of someone just graduating college, see this: https://earlyretirementnow.com/2016/12/07/the-ultimate-guide...
That's a big assumption, and doesn't take into account the reduced spending power (cost inflation) during that time.