While I understand the points made here and the article's points - if a person's income fluctuates or is uncertain - I would think having no debt would be a big plus. True, you are missing out on investment interest but in case of another Great Recession but if you are debt free it takes less money to survive or a smaller job to keep the lights on.
MUCH easier if a person doesn't have a house payment or some other kind of debt.
The assumption is that the market will always go up and historically it has over the long term. However, over shorter periods like a five year period or a ten year period - a person could be a dire straits while trying to float debts, investments have tanked so you can't continue to be FIRE'd, low or no income (unemployment or underemployment), etc. It might be difficult to return to work after retirement if your skillset has is views as out of date. Age discrimination is real. Wal-Mart job? Sure. Return to professional work? Not as easy.
Who wants to be in their 70s and and lose their home or be forced to return to work when their body is failing them due to old age b/c the economy tanked?
Choices made at 30 yrs old can be very different than choices at 70 yrs old. I've witnessed the retiree with a mortgage. Witnessed another person on the cusp of retirement only to have the economy belly flop (2008) losing a big part of their investments when a sizeable number of investments evaporated with the market failures. This delayed retirement by a several years (delaying other people's ability to advance in their own careers) and significantly changed the kind of retirement they could afford.
