The reason networks are all shutting down 3G is because very few people use it anymore, which means the 3G bandwidth is split between 10s of people whereas the 4G is split between 10s of thousands, as is 5G
250Mb/s among 10 people is 25Mb/s for each person, 10Gb/s among 10,000 people is 1Mb/s for each person (made up numbers and it’s a bit more complex than this, but it demonstrates the point), so even though the 5G & 4G are capable of transmitting more data, per person they’re not.
If you repurpose the 3G tower as 4G or 5G you can cut that 10,000 in half, which annoyingly gets rid of the hack to use 3G when it’s being slow, but does improve the speed for most people