[patchwork]

Sure. Consider the example of Roman concrete; there were three concrete factories in the Roman Empire, which supplied the needs of the whole Empire, with their buyers overwhelmingly being government. The empire collapses, trade networks fragment, and now you can only reliably sell to the province you're located in, and one province doesn't have the demand to keep a factory going, so all three factories shutter for lack of demand and the technique for making concrete is forgotten.

High technology frequently requires a large market to make the degree of specialization required profitable. When the market fragments, the people and specialized equipment necessary for cutting-edge tech will find itself swiftly repurposed, and techniques and data get lost because preserving them is expense for no revenue.

Having said that, it's a fair question how far down you could really go. A worldwide depression shaving off one TL is extremely plausible, maybe even demonstrable. Heck, look up "institutional archaeology" if you're unfamiliar with the term - technology companies already have to sink time and effort into fighting this, as market and employment shifts destroy knowledge networks that cannot readily be rebuilt from scratch. Could we get two TLs? I think we could, if the energy sector was badly disrupted. The price of electricity and/or oil goes through the roof, and suddenly lots of things become commercially nonviable, shipping is disrupted, starvation becomes a possibility, the Internet becomes an intermittent and possibly local phenomenon...three? I'm struggling to see how you get three, although it's fair to say that such an economic collapse almost certainly causes the wars and plagues you were trying not to posit.