You dont NEED alts.
Yes you make alot more gold and can pool mats and stuff to your main to progress it further a lot easier. But not neccessary.
I started playing on release with only 1 character and Im at 1480 and make gold just fine.
Ive only just recently created 3 more T3 characters as of last week.
It all depends on if you want to play casually or if you want to grind and do the tasks on alts as well and get higher levels faster / more gold faster.
Alts = more mats, more gold, faster progression.
No alts = you still get there in the end, its just slower.
And yes you wont start actually making any decent gold till probably T3.