Why does global suspension of economic activity result in catastrophic loss/debt? Covid-19 has forced a suspension of at least 50% of financial transactions and business activities. 

Materially, not much has been lost: consumables have been spared, equipment is dormant, maintenance is mostly inexpensive. Why is the health crisis expected to bankrupt many businesses? Is most of the loss speculative?

Propery tax + land rental = 20-25% of yearly rent, so a landlord can waive rent for 6 months and still make profit! enough to pay 2 years of tax and maintenance. 

I naively imagine that using government moratoriums on bank-debt-interest rates and fees, It is possible for companies to organize a state of dormancy and maintenance for an extended period, without incurring significant loss. 

Can you give a summary of why it is not so simple, and why the losses are a lot worse than a financial and material dormancy?
It creates losses. 

First, many businesses don’t own their own building/land/equipment. It’s very common to lease these things. Moreover, many business take out insurance that again has to be paid on monthly basis. In Europe in many countries you need to pay workers certain portion of wages even when you can’t call worker to work or even if they stay home sick. 

The list here is not exclusive. The point is many business have non-trivial fixed costs that have to be payed whether business operates or it doesn’t. Furthermore, in short run even some variable costs - like wages to employees on production line are fixed (and in many countries this can be up to several months). Hence, it’s not like a dormant business have no costs. 

You might ask why then we don’t suspend for example these leases etc. but that just moves the problem to the different parties and does not eliminate this issue. 

Moreover, very large business might have enough financial cushion to absorb losses for few months, but there are many small-medium size business that can’t do that. For example, consider a sole trader (a form of business organization where one person embodies a business) who has a repair shop with no employees just living month to month. That kind of businessman can not afford to go on hiatus for very long.

Furthermore, if by long you mean not just weeks but several months or even year that would be issue even for the best cushioned business.

So yes the impact could be catastrophic. For example according to this VOX blog the impact under some scenarios could be worse than recent 2008 crisis.