Nobody is suggesting this. How did you come up with that number anyway?
There are some deals that have been posted on the forum recently for 8TB drives for $200 or less. One 8TB drive can replace 4 of your 2TB drives. 7 x 8TB could replace all 28 x 2TB. Then you would only have 7 drives to power, 7 drives to troubleshoot, 7 drives that might fail instead of 28.
In fact, you could convert one of your parity to data since you would only have 7 data disks, and just buy 6 x 8TB. And as mentioned, there is no need to do it all at once.
So, you could do the complete conversion for about 1.2K, and the expenses and troubles of finding cases and power supplies to support so many drives would be gone.
You mention using this system for graphics work. Did you know that unRAID doesn't stripe data? There is no performance benefit from multiple drives, since any file is read from only one drive.