That's usually true
if you see the actual costs of the thing.
In the case of data transmission if you were using some data transmission system where you were charged per byte based on the operator's costs (possibly time dependent so the cost per byte might vary depending on the amount of traffic on the provider's network and on the varying real time prices of the networks they connect to) then you would indeed see prices going down over time and performance going up.
Consumers, small businesses, and often even medium businesses generally hate that kind of pricing. They like fixed monthly bills. So providers offer that, setting the amount of data included in that price high enough that most customers won't ever come near it.
That tends to result in the lower bandwidth users actually paying quite a bit more than they would if they had per byte pricing and the higher bandwidth (but not so high as to go over the included data and hit overage fees) paying less than they would under the per byte model.
That can attract more high bandwidth customers and eventually the model of customer bandwidth usage that was used to set the price and bandwidth allowance is no longer accurate and gets adjusted.
Note that this means that price you pay is not just a function of the underlying technology costs--it is also a function of how other people are using the service.
Same thing happens even in non-technology areas. You probably wouldn't go into a fixed price "all you can eat" restaurant just to get a donut and cup of coffee. The fixed price is set to cover people getting full meals. And if a bunch of competitive eaters started coming in every day to do their training at that restaurant you can safely bet that the price is going to go up for everyone or there is going to be an asterisk added to "all you can eat" with a footnote that puts some sort of cap on it.