Let's say that this hobby is going to cost $20k per year (I'm betting you can do way, way better). That's an extra half million dollars to need in stache to retire. Not trivial, but doable. I don't know about you, but I'd happily work an extra 5-6 years (likely less because now compounding starts to really help out) for the right girl. And if in the interim she could take a break from the hobby for a year or two, to really supercharge the savings, like say, while popping out some chittlins, that's cool too.
Sure he can do better - he's at zero! It's how much she'll cut back that is the question. :(
But, dayum! $500k in 5 year is over $80k/year saved. I don't know what you do, but that's essentially my whole paycheck and I'm pretty sure the tax man intends to get his cut and won't accept "but my partner really likes horses" as an excuse.
So for a specific expense, when you talk about how much it will delay FIRE, the thing to remember is that the theoretical FIRE date sans-hobby is not going to be delayed as much as it would seem.
A huge part of the "saving" that you have a hard time envisioning at that point is going to come from delayed withdrawals on your already FIRE-sized stash.
Taking the case of 50k/yr take-home pay, 7% annual returns, 3% annual growth of salary, and a 70% savings rate, the first 500k takes 10 years to earn. But the second 500k only takes 5 years.
70% saved 7% returns 3%/yr raise
Year Total Stash Passive Earnings Earned Income
1 $35,000.00 $2,450.00 $50,000.00
2 $72,450.00 $5,071.50 $51,500.00
3 $113,571.50 $7,950.01 $53,045.00
4 $158,653.01 $11,105.71 $54,636.35
5 $208,004.16 $14,560.29 $56,275.44
6 $261,957.26 $18,337.01 $57,963.70
7 $320,868.86 $22,460.82 $59,702.61
8 $385,121.51 $26,958.51 $61,493.69
9 $455,125.60 $31,858.79 $63,338.50
10 $531,321.35 $37,192.49 $65,238.66
11 $614,180.90 $42,992.66 $67,195.82
12 $704,210.64 $49,294.74 $69,211.69
13 $801,953.57 $56,136.75 $71,288.04
14 $907,991.95 $63,559.44 $73,426.69
15 $1,022,950.07 $71,606.50 $75,629.49
A huge part of the savings each year becomes the returns on the previous year's stash.
Of course, if the expensive hobby starts to reduce the total savings amount, things start to look catastrophic, thus the points about monetizing the hobby and making it as efficient as possible.
This is a big part of why one-more-year is so attractive. It's not just the additional savings from one more year, it's deferring the withdrawals.