^^^

And I believe the Wii U discs could hold up to like 50 gb and the only game that actualy came close to filling that was Xenoblade X I think.

Wow so only half. Well, Nintendo apparently is fantastic at optimizing games and the space they require. I know people on this forum have gone crazy at how small Nintendo games are memory wise when they actually play the games and they feel very large and detailed for the memory size. Needless to say, if they go cartridges, I doubt they will need anything larger than 64gb sd card (if that) for each game if memory in those things works the same as it does in discs.

At some point in the future, perhaps, but not quickly. The genesis had a 400 KB/s transfer rate for most games sizing 2 MB. It means the entire cartridge could be theoretically loaded in 5 seconds. To achieve a equivalent result, we need 12 GB/s, which is the current DDR4 speed, and that's 30 times more than current common SSD speed. To put it in perspective, 30 times is about the speed factor between the 1997 PC133 SD-RAM and the 2016 DDR4.

Streaming, perhaps, perhaps not, but as for asset streaming, there is a long way to go if the goal is to go back to the near instant 90's loading times.

About the OP question, "not much more expensive" is not that relevant. A BR is about 1$. If the cartridge is 2$, it's a few percent less profits (if we assume 10$ profit for the editor, it's 10% less profits)... they care. I don't know about the cost of a ROM nowaday, I have never heard about 64 GB ROM, let alone about a mass market for big ROM that would drive price down. But, let say it's exactly the same price as a BR, and let's say also the transfer rate is excellent (which is far from proven), with the massive updates, DLC, your game loading would massively slow down, as a developer you could not even rely on better transfer.

The problem is how you are looking at it. You arent thinking big enough.

First off, look at internal drives. And not HDDs or SSDs. I'm talking M.2 drives.

Those can offer transfer speeds (esecially in an optimized console enviroment) of at least 3GB/s. That is magnitudes better than what you can get from the fastest cart right now (~250MB/s) and  better than what you can get from any SATA drive. 

Now why that is important is that if you can see that, then you will see that all  console manufacturers have to focus on is getting the game into your box. Be it via a disc drive or via download. Cause all the games will be run off the internal drive at the end. Thats already happening now.

Basically, the best and cheapest kinda "cart" to have is an M.2 drive as that isnt just better performing than acart but also can hold waaaay more. There is even a 16TB m.2 drive out there now. And as long as all a disc or cart represents is just a way of moving data to your console, then a disc will always be cheaper than a cart. 

Someone in Sony, MS, and Nintendo have crunched the numbers and decided that discs are the way to go. If cartridges were cheaper we would see them.

Comcast, which has >50% share of broadband internet in the US has a datacap of 300GB a month.   They are looking at raising it to 1TB, but thats only in 'trial markets'.

Isn't the cooling for the heat generated by the CPU and GPU rather than the optical drive?

All I know is there are plenty of optical drive free devices that need additional cooling :p

"Cartridges are not much more expensive to produce than disks", LOL ...

Console manufacturers would know more than some random commentator at mynintendonews. Yes, a disc drive can cost more than some integrated cartridge slot with the memory controller but with cartridges you risk having to pass on the costs to publishers, consumers, and console manufacturers instead of passing it on to the platform ...

As far as faster read/write throughput or durability all of that is dependent on the quality of the entire peripheral system so you can't just expect any random solid state based storage to beat discs in these metrics ...

Of course it will. As of right now you can already find M.2 drives for as low as under $300 for 500GB drives. That would have been u heard of jist at the starr of rhis year. By 2020/2021 I can pretty much gaurantee that we can habe 1TB M.2 drives for under $80. And thats not whay sony/Ms/nintemdo will pay for them.