Which packages is mlt7 going to break?

I see kdenlive and shotcut in Factory, potentially.

Can't they just be updated

Perhaps, but they are quite large and I'm not the maintainer.

. Why is an mlt update needed.

It's not "needed", but it's out. It's often a chicken-egg question when it comes to libraries.

What do you suggest?

Ok, I've inquired. Shotcut can be updated and actually needs mlt7 to work. kdenlive won't be ready for quite some time.

So, if mlt is not updated, it will not be possible to keep shotcut up to date. If mlt is updated, kdenlive will stop working.

I must insist, see sr#891248