If your team isn't delivering on weekly sprints something is wrong. Perhaps you are over committing. If you are resorting to two week springs to forgo the useless double hour loss of the sprint plan / follow up demo, then sure, I get it.

In any of the two above cases it sounds like you have a weak team on your hands. When you push to treat the subject like a marathon it makes the business people nervous.

When you are on an incremental weekly delivery cycle, and things are late, it is usually only late by one or two of those weekly cycles. BUT, If things are late on a "marathon cycle" they are usually late by a "marathon length".

You sort of have to keep in mind that there's a rather large reason that there was a push by the business people to force faster cycles. You and your team are expensive. Weekly or bi-weekly goals show your accounting department that they are receiving assets.