None of this us a rebuttal to the argument that owning things isn't a job.

the capital that is in the building would be used for something other than the building.

If the landlords don't exist, they don't have capital to invest, yes. That's not to say building won't happen without them taking their cut. Either the would be occupants could build, or (better yet) we decommodify these essentials and have them publicly held. We know how well comodifying the essentials goes and how that's working out for peoples' ability to keep a roof over their heads.

The manager has done a similar trade, just with their time that they'd otherwise be doing something else with.

Management is work, so I'll not muddy the waters - the business owner isn't working, and is taking the value of the workers' labour. The fact that they have amassed the capital to be able to do this whole the workers can't isn't an argument for simply owning things to be profitable - it's an argument for workers to get a fairer slice of their contributions. They should own the business, but the capital hurdles prevent that.

In the long term, that's what the landlord hopes for, that reducing their standard of living in the now will make them better off in the long run.

You'll have to forgive me for not losing any sleep over the fact that non-productive owners assume a trivial level of risk to extract near guaranteed profit for no work.

You asserted that landlords don't do anything. I'm pointing out what they do -- they're the reason that the building is there.

Workers built the building, workers will occupy it and give it reason to exist - the owner just owns it - this isn't a necessary part of the equation for any reason other than the fact that you can amass unlimited wealth by simply owning things, inflating prices, and draining resources from the productive members of the economy, pushing them out of being able to buy the property they'll use so that they can buy the property, extract more wealth and perpetuate the predatory cycle.