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Once again, this form has been going on since 2013.
Here are, once again, my opinions on this;

A few years ago, I made a post here, explaining how this would be a horrible idea. And I stand with what my original post had written, it would be chaos!
If scratch were to convert all projects to 16:9, everything would break and look bad, as all projects were built with 4:3 in mind. Even if scratch team were to make it a toggleable option, people would still soon begin to migrate entirely to 16:9, and overall begin to prefer it. Leaving the poor 4:3 projects into dust as the child creators either don't want to, or don't know how to convert them into a 16:9 project. This would inevitably make everyone switch to 16:9 and 4:3 being removed in a later update due to inactivity.
Adding on, 4:3 is clean, simple and neat. The point of teaching programming to children on scratch is to have it be clean, simple, neat and easy to understand. A toggle would be confusing for younger kids, as they wouldn't know why their teachers screen is in 16:9 but theirs are in 4:3. So they'd all get confused and switch to the FAR inferior, Scratch JR, or even worse, the paid Tynker. Deserting the entire scratch website for probably indefinitely. (thanks to TheSmartGuy1234 for that)
Finally, have you ever heard of project storage limit? Projects can store only 50mb of code and 10mb of assets (sprites, costumes, music, etc), so unless they higher this limit (very unlikely) 16:9 would be incredibly awful for the 10mb asset storage limit, as you would need more space to draw your sprites, and thus, more storage usage.
People definitely have mixed feelings about this topic, and I respect your personal opinions on this matter, but my opinion is sealed indefinitely, as I am (most likely) leaving this discussion forever. Thank you for reading.

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