I think the problem, OP, is cost.
There is a good deal of data that has to be stored for your commander. Faction reps, ships, loadouts, mats, ranks, etc. One more CMDR doesn't seem like much, but if 100k players had one more CMDR, the increase could be significant. FD has to pay for all of that storage, and if we're using two CMDR profiles on one paid account, they are doubling overhead without increasing income.
Now, if this were subscription type thing, I'd say yeah, give us more account options.
That's my take on it, anyway.
There are many games out there that are F2P that have multiple accounts. They also have tonnes of data storage for each character but several of them that I have played have 8 slots for characters.
FD has a store page so they are gaining on both ends of the scale, charging for the game, (the F2P games are free downloads) AND a store to make more income so they potentially make a lot more than most F2P games. Stop making excuses for a corporate body whose main interest is making as much money as possible in the shortest amount of time so the bosses can buy the latest yacht.
As a programmer myself, I'd be interested to know exactly how much data is kept per character because I honestly don't think it's as much as you believe it is. Database compression methods are good these days and hard disks have more storage than the average home user could ever use now, just one disk alone can hold over 10 Terabytes, (that's 10 thousand gigabytes 10x10^13). I know people who are still using 500gb disks and still have not filled them up, every game I own (and I own a fair few) still comes up short of 1Tb.
All the data that is specific to your character and is only accessible by your character probably amounts to 1 or 2 megabytes at most, the rest is public database stuff (What Cmdr discovered which system etc). You have a database of ships, fittings and engineer upgrades and what ships you own only needs to have a byte for each ship to tell it which database entry number that ship is, the same goes for the fittings and upgrades. In total, I'd say at most, your data for each ship is no more than a couple of Kb. Practically everything about your commander relates to single numbers that point to records in public databases. Let's be generous and say 5mb per commander for arguments sake, now let's say 100,000 commanders currently playing the game, that's a paltry 500Gb of data. Even a 1tb hard disk is only 50 quid and that would hold twice as much. Are FD THAT hard up that they can't afford 50 quid and allow us an extra 1 character each?
The reason I stopped supporting SC was because they got greedy. With the amount of money they've made so far, they could afford a super massive team to develop it and the game should have been completed (and optimised a lot more than it is at present) long ago, (plus the ships they are selling are just way too expensive and also make the game Pay2Win).
DB appears to be going the same way since he still has not kept his promise of a single player offline version of this game he made when it was still a kickstarter project. At least with single player offline, you could save your game and start a new one, then go back to your previous save game later. This has always been a great feature of the original Elite games, you can start over anytime but still go back to your previous character and continue where you left off. Not everyone wants to do the same thing in a game day after day when the game has so many other things you could do, but you can't go pirating because it affects the standings of your character which means you could then be limited when you go back to what you were doing before.