Why isn't a previous living world season unlocked with the expansions?

Because money.

penalizing you for not being a player back in the days.

Not at all. It's rewarding those, who played back in the days :)

Lastly, there's just the thing of maybe a lack of grouping of new people when it comes to fractals, raids, wvw and some other modes, as most of the people running these are sort of barring the LFG only for players that know what they are doing and I can understand that. However, I guess joining a noob friendly guild that would take the time to teach you those sorts of things might fix that to a point.

That's on you to find likeminded players and/or a guild. You can also start your own "newbie" LFG group.

Most of your complains regarding HoT are being repeated since 2015. At this point I don't think it's going to be changed so you better adapt.