The iMac which first went on sale in 1998, helped revive Apple which was almost on the heel of dying. Of course, the iPod helped and later the iPhone just skyrocketed the company. However, the iMac is still one of the most important Apple products as it is used by most iOS and iPadOS developers to create new apps.
The iMac has been rated the #1 desktop computer in the US multiple times. However, the machine has not received major redesigns since 2009. Apple did cut down the thickness of the iMac in 2012 when it ditched the DVD Drive (SuperDrive) and also started adding SSDs with its Fusion Drive (HDD+SSD) technology.

A 23″ budget offering in the works
For years now, the internet has been swirling with rumours of a completely refreshed iMac with a new design. The rumour mill now suggests that a new 23” iMac is under work for a release in the fourth quarter in 2020 with mass production beginning in the third quarter of 2020. It has also been suggested that the 23” machine will be a budget model to appease to the mass market.
Apple has begun relying on practices of reducing the price of its products by a tad to increase sales as the company looks forward to making more money from its services businesses. If the Mac lineup gets cheaper, it will be much easier for the company to lock people in its ecosystem of apps and services.
The rumours suggest that the increased screen size will not change the dimensions of the iMac as Apple will cut the huge bezels on the front. There have also been rumours from different sources (not the same as the 23” iMac rumour sources) that the company will introduce FaceID on the iMac in the future.
It is clear that 2020 will the year when Apple releases new iMac but none of the specifics can be confirmed at the moment.