Questions about using an old iMac as a monitor for a new iMac

Using an iMac as a monitor - Apple Community directly answers your question. It also includes links to other options you may consider including the Luna Display device.

Specific answers are as follows.

1: From their website: https://shop.astropad.com/products/luna-display

• Can I use Luna Display for gaming or video playback? → Luna Display is intended to use as a side reference screen with content that doesn’t frequently change. Luna is NOT recommended for gaming, video playback, or other high-motion content.

Pay particular attention to the references to "content that doesn’t frequently change" and "video playback" then draw own conclusions as to its suitability for your proposed purposes.

2: no

3: No one really knows because it would take decades to determine an "average" lifespan. Having said that none of mine have ever failed, the oldest of which is approaching 40 years of age, but it uses a CRT. They are not supposed to last as long as LCDs. With the exception of certain life-limited components that aren't used any more anyway, Macs typically become functionally obsolete well before their hardware fails.

silogen53 wrote:

1) Could using Luna and my 27 inch IMac to monitor lead to any lagging in terms of video compared to just doing everything on my 24 inch IMac without Luna?

Astropad – Astro Blog – Use Your iMac as a Monitor: A Step-by-Step Guide [2025]

"Like any product, Luna Display does have limitations and it’s not recommended for high-motion content. For example, I wouldn’t recommend using Luna Display for gaming or video editing.

Since Luna Display is squeezing a 5K display through a USB cable, when lots of motion occurs on-screen (like from video playback or moving a window quickly) the screen can temporarily become pixelated before it renders in at full resolution."

Note the explicit mention of video playback.

2) My 2020 IMac will presumably soon stop getting software and security updates. If I'm just using the 2020 IMac as an external monitor, is there somehow some risk of transferring some infection on the 2020 IMac to the 2024 iMac?

No.

3) What is the average lifespan for IMac displays? I've heard something like a general rule of 60,000 hrs of use for modern monitors (which would be around 20 years if you're using the monitor for an average of 8 hrs per day). Can I really expect an IMac monitor to last that long? Again, I'm just talking about the display/monitor; the rest of the computer doesn't matter given my purposes.

The rest of the computer does matter, because if the rest of the computer stops working, you won't be able to use the display.

Workarounds like Luna Display do not turn your iMac into a dedicated hardware monitor with a special path from an external port to the LCD panel, one that bypasses the rest of the computer. They run stuff, in software, through the rest of the computer. That's a big part of why these workarounds have lags and artifacts – and it also means that, if the rest of the iMac stops working, it won't be able to support Luna Display.

Hi,

2020 iMac doesn't support Target Display Mode.

Use your iMac as a display with target display mode - Apple Support