Why the Sidecar doesn't work on MBP Retina, 15-inch, Mid 2015?

FWIW, I misread that post, I thought is said the 2015 was compatible. Haven't had my coffee. It has nothing to do with how long a MBP has been sold or a "marketing" move. A 2015 MBP is still a 2015 MBP no matter when it's sold.

You can let Apple you are disappointed here: Feedback - macOS - Apple

Sidecar-supported Mac models:

• MacBook introduced in 2016 or later

• MacBook Air introduced in 2018 or later

• Mac mini introduced in 2018 or later

• iMac introduced in late 2015 or later

Yours doesn’t appear to be supported

• MacBook Pro introduced in 2016 or later

• iMac Pro introduced in 2017 or later

• Mac Pro introduced in 2019

https://www.apple.com/macos/catalina/docs/Sidecar_Tech_Brief_Oct_2019.pdf

No., the 2015 MacBook Pro is not compatible. Apple has updated the Side Car support document, to specify exactly which Macs its compatible with though it provides no insight into why certain models compatible with Catalina are left out.

See here: Use your iPad as a second display for your Mac with Sidecar - Apple Support

Sidecar requires Mac hardware support for HEVC/H.265 video encoding, even for a machine with a discrete GPU (which it does not use), and without a T2 co-processor. A 2016 MacBook Pro or later is needed because of two factors.

1. Intel's Quick Sync needs Skylake or later processors for the above encoding issue
2. Machines with a T2 chip will do the above encoding without needing Intel Quick Sync.

1. How would Apple retrofit older Macs with a minimum requirement Intel CPU generation (Skylake) that was the first to support CPU hardware encoding of HEVC/H.265 via Intel Quick Sync?
2. How would they retrofit T2 chips in older Macs to provide the hardware HEVC/H.265 encoding that is not done on the slower GPU?
3. How would they backport the Catalina FaceTime which uses this HEVC/H.265 encoding for the best A/V bandwidth to supported iPads — to older Macs that cannot support HEVC/H.265 anyway?

Apple had to make a decision that the majority of users would prefer brisk Sidecar performance, even if it left other users with unsupported hardware behind. For unsupported users, there is still Duet.

What iPad do you have?

Sidecar-supported iPad models:

• 12.9-inch iPad Pro

• 11-inch iPad Pro

• 10.5-inch iPad Pro

• 9.7-inch iPad Pro

• iPad (6th generation or later)

• iPad mini (5th generation)

• iPad Air (3rd generation)

I just want to add my voice in this. I do often respect apples hardware limitations to new products as it allows those that are within specs to work quite well. I do feel disappointed at this moment though as sidecar is a feature I am super excited about. It feels strange to have developed this with only future models in mind and leave some people like myself who don't necessarily have the thousands of dollars for an upgrade right now, in the dust. Does it make me want to consider getting a new laptop, yes! But do I like that, not at all.

I know Apple won't likely ever support the configuration I have and I guess for those of us who are excited and left behind, we'll be on duets or astropad till we upgrade.

From hardware aspect, early 2015 is more than enough to Sidecar?

2019 MBP 15" SPEC...ahh

Then I don't know. I don't use sidecar. A configuration issue possibly.

See if this article helps. https://www.techradar.com/how-to/how-to-use-sidecar-in-macos-catalina

11-inch iPad Pro