I'd think most offices can't be converted to attractive multifamily in a cost-effective manner. Hotels would be a more natural conversion, but even then you'll have a multitude of issues (zoning, location, conversion costs, hotel demand in your market) and it's probably not worth it.
Commercial tenants typically pay their utilities, but for this building it sounds like there's so many tenants that it would be a nightmare to handle billbacks. An all-in lease rate makes more sense from what I'm gathering.
The space you're looking at just depends on the numbers. I assume either you're looking at a larger building or it's an office share-type arrangement where individuals and small businesses are renting small offices/suites/desks. Either one can be attractive because you've got a lot of tenants, so your risk is spread out and it's not all dependent on one major tenant (I'm assuming).
Current office trends are a little worrying if you're an investor. There are signs of the market picking back up, but the general thought is your typical office user is going to want ~20% less square footage now than they used to if we're looking into the future. Doesn't mean offices are a bad investment, but people just aren't coming to the office like they used to. That could change over time, but a lot of companies are probably only at 30% capacity relative to their pre-pandemic levels.
That shouldn't scare you from buying entirely because what matters most is the terms of the lease and the credit/security of the tenant. That's where the building's value comes from. Vacant offices are now much more risky than they used to be. Could be some opportunity to buy at a discount if the office comes back strong. I'm pretty sure we're at the bottom of the office market right now and my theory is that it will recover. Question is how much it recovers compared to where it was at before March 2020.