> Keeping it vacant only impact current income, lowering rent impacts future forecasts.

Does it though? Suppose you can't find a tenant right now because the market is soft but is predicted to improve in a few years. If you leave the unit vacant, you lose money right now. If you rent it out with e.g. a 3-year lease, you make more for the next 3 years than you would with a vacancy, and if the market price has increased by then you can increase the rent on the unit and either get it from the current occupant or the one you get to replace them in the high demand market when the higher rent causes the low-paying tenant to not renew the lease.

So taking a tenant now only improves prospects (you fill a current vacancy) with no negative impact on future returns. The only thing it does is imply that current rents are lower than before and future rents might be too, but a vacancy implies that even more strongly.