The option to go for a command on the freighter does not then prevent you from going back onto the pax fleet at a later date. You have a CX seniority number, and when your seniority allows you to be eligible for command then you can bid for it. The reason that the freighter commands can go to quite junior people is varied. For a start, the freighter command will be on a base and in the case of a US base, there aren't alot of people in CX who can live in the US or Canada. Therefore, the pool of people you are potentially competeing with for that command slot are alot less than if you are just wanting to take up your earliest command slot in HKG. Secondly, the pay scale is less than that of a B scale pax captain (although it has improved a little recently). Thirdly, you are then tied to flying the freighter only for 3 years (min). Now for those who are bidding for and then successful on their command course having only been in the company for 3 or so years, due to their relative seniority, they will have to end up staying on the freighter for probably in the order of 5-6 years before being able to bid back onto a pax command slot. Ifthis doesn't worry you or whatever, then go for it. The freighter roster isn't great so being tied to that for 6 years may be something that stops some from bidding for it. I think for some, there is still a stigma attached to being a "freighter" pilot only due to how the freight thing was set up with ASL years ago. Still for others, they may not even be aware of the possibility of bidding for a freighter slot. etc etc etc.
What some may not be aware of is that once you are in the company as an SO, there is nothing stopping you from bidding for a freighter FO vacancy on a base. eg. you are a Canuck who joined as an SO living a based in HKG. You are thinking that the earliest you will be able to be on a base is probably 5 or so years away due to your time as an SO then JFO etc. before finally being able to get on the 400 or CCQ'd on the Bus and able to take up a base. Well, a notice may come out seeking any interested pilots to take up a freighter base in LAX as an FO. There is nothing stopping you as an SO putting a bid in for that slot. If successful, you'll then do your training onto the 400 as an FO and take up the base. After your 3 years or so on the freighter, you'll more than likely be able to bid straight onto a pax 400 base in the US. Now, when you start thinking about command, if moving to HKG for your command on the pax fleet via the 777 or 330 isn't what you'd like, then obviously bidding for that freighter command on a base will be your option again.
Hopefully this explains a bit about how upgrades can work. One note I need to make in all this is that you don't just get a command course automatically. You can bid for it but then there is a bit of a selection process that goes on as your file is looked over etc. There are plenty of guys who have been Cat B'd (ie. not suitable for command at the moment....we'll check up on you in a year or so to see how things are going)