I'm coming at this from the perspective that they will either release two consumer-level sizes of displays that will match the current and rumored iMac display sizes, or they will just release the larger version. I personally don't see Apple releasing just a 24in external display.
Why we are getting new external displays at Monday's event:
• It seems plausible that new iMacs will be announced. The phrase "Big Things" has been used to promote the current iMac line. It's also being used to promote the event on Monday. Perhaps a 30in (or larger) display size?
• I believe the 24in LG Thunderbolt display was discontinued and possibly the
32in 27in 5K one as well. Neither one are available from LG.com.
• The
32in 27in LG display is only sporadicly available at West Coast Apple stores. It doesn't seem to be available anywhere east of the Mississippi.
• Both LG displays are showing at least a 1-2 week delay in shipping.
Why we are not getting new external displays on Monday:
• The 27in iMac is very well stocked at pretty much all Apple stores, so a replacement seems unlikely. Thus a larger external display seems unlikely.
• The delays with the LG monitor could be nothing more than a supply chain issue.
• I've learned never to get my hopes up about a new external display.
The other possibility is that they will announce (via press release) another partnership with LG or someone to provide displays. Which...ugh...I think the LG displays are so ugly.
The next question...would the new displays have a notch?
Wait...Nevermind. I don't want to know.
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Edited to change 32in to 27in. The LG 5K monitors were 27in instead of 32in. Thank you @xraydoc for pointing that out!