I would not call the two orders of magnitude difference that you can get between true native and this new webcrapp stuff "slight". There are reports of Teams taking dozens of gigabytes of RAM --- that's just not something you would see with a native application with the equivalent functionality, unless it was written extremely horribly. Dozens of MB is a comfortable amount of memory for such an application to use, and the old MSN Messenger (native! with many third-party clients too!) was in that ballpark. Remember that Skype used to have a native Linux client too. Microsoft is a huge company. They can certainly afford it.

is textbook waste of money.

So it all comes back to corporate greed. Inflicting a dismal user experience on everyone, for what would be an essentially tiny savings on their end.