Taxes are one of the most powerful incentives in the world. For instance, dumping positions at loss before new years eve might help you with your tax bill since you have losses. And of course, setting up a company in a country where you did buy a lot of spectrum and you have huge losses, effectively creating a tax credit ( https://www.reuters.com/article/telefonica-germany/update-2-... )
Selling stocks at a loss is only helpful in the short term, or if you don’t think those stocks are going back up. If you harvest a loss for a tax deduction on December 31, and the stock goes back up by the end of January so you’d be even, you are way behind – because the loss only comes back to you at your highest tax rate.
For instance, I could sell my losses in a growth fund only to buy a different growth fund (not the same index though) and I get to stay in the market.
Ideally you would do this on day 364 of losses to maximize the tax incentive.