Some upsides with multiple jobs per class are:

Possibility for more jobs with weapons similar to the ones we have now.
If all jobs are disconnected then to introduce another job that uses bows then you would have to create a new specific type of bow that is unique to that job.
This doesn't work as well if the jobs have different roles though since the main and substats might differ.

More jobs implemented overall since classes to build upon already exists.
It probably takes shorter time to build a job on a class that already exists than to create a new class and job with all new abilities.

Downsides would include the risque of the jobs feeling way to similar unless done right and lack of investment as a player if you already have the class at max level and only have to do the job quests.

I would love if they could at least make it so that adding more jobs to existing classes is a possibility in the future. It is always nice to have options when creating new jobs after all.

That being said, I think they're going all in on only making new jobs that have no connections to the existing classes since that's what they've said that they will do.