I think it all depends. Your $60k of existing losses are enough to last for 20 years of not selling anything for a gain. If you plan to be retired to a low tax bracket before then, and you don't plan to liquidate a bunch of stock at some point prior to retirement to buy a house or something of that nature, you're right that harvesting more losses probably won't be a major win.
Let's look at an example. You say you have $60k of existing losses, and could harvest $10k more. Let's say that $10k unrealized loss comes from stock you paid $100k for. It's worth $90k now. Let's suppose you want to start selling that stock in a decade. It appreciates to $200k by then, and you want to withdraw $50k per year. For the sake of easier calculations let's suppose the market stays flat for a few years after that: you withdraw $50k for four years and that uses up this lot of shares. In the ten years between now and then you are able to use $3k of your existing loss per year to offset other income, leaving you with $30k carried forward in a decade.
Scenario 1: You don't harvest your $10k loss now.
Year 1 of withdrawals: $3,000 net capital loss. You start with $200k of stock, $100k cost basis. You sell $50k, realizing a $25k gain. Your existing $30k loss carryover wipes out this gain and you're able to claim an additional $3k against regular income (Roth conversions, perhaps?).
Year 2 of withdrawals: $23,000 net capital gain. You start with $150k of stock, $75k cost basis. You sell $50k, $25k gain. Subtract your $2,000 of loss carried forward, and that leaves you with a $23k gain and nothing to carry forward.
Years 3-4: $25,000 net capital gain.
Scenario 2: You harvest the $10k loss now.
Year 1: $3,000 net capital loss. You start with $200k of stock, $90k cost basis, $40k loss carryover. You sell $50k, for a $27.5k capital gain. Your carried-forward losses erase this gain plus $3,000, leaving $9,500 of losses to carry forward.
Year 2: $18,000 net capital gain. $27,500 gain minus $9,500 carry-forward. Nothing to carry forward to the next year.
Years 3-4: $27,500 net capital gain.
Not a huge difference here. By harvesting $10k of losses now you have $5,000 less capital gains income in year 2, at the expense of $5,000 more capital gains income spread evenly across years 3-4. What this would do is give you a bit more flexibility on the timing of your income. If you really wanted to keep that capital gains income lower in years 1-2 you'd be able to follow the withdrawal schedule I outlined above for Scenario 2. If you wanted to smooth out your income a bit more (for ACA purposes or something else) there's nothing stopping you from harvesting some gains in Years 1-2 to let you have lower gains in years 3-4. With a smaller amount of carried-forward loss you lose a bit of this flexibility, but it likely won't make a huge difference either way.
