In Tekken 8 the Single Player is not bad, I estimate that the Main Story and all other character episodes will keep you busy for about 4/5 hours (depending of course on your ‘skills’ and whether or not you have all want to watch videos).
After that the focus is more on Multiplayer. The Tekken ball mode is fun, but it’s better to play with a buddy. I don’t know how much you want to ‘costumize’ your characters (give them different clothes, etc.), it’s also fun and can fill you a few hours, but don’t think that should be your goal in a fighting game.
However, I think that the developers have done a very good job to address your ‘fear’ (and others who think it is too high an entry point) to play multiplayer.
For example, there is an arcade quest that teaches you the tricks of the game. In the same arcade quest you can also download ‘ghost’ from other players and play against them to see what the approximate level of the player is (as also described in the review). That all helps you get used to the level.
If you want to play more from people of your level, my advice is to play ‘ranked matches’. sounds a bit strange, playing for ‘official points’. but in ranked matches you will encounter more people of your own level (of course you must first have played an x number of games to have a good idea of what your level is).
Maybe I’m a bit biased myself, because since Tekken 3 I’ve played almost all Tekken series (except for Tekken Tag 2). I must say that the level, especially at the beginning ranks (1 to 6), is very doable. Of course there will be (some) very good players, but those are more of an exception. In the early ranks you will rarely see such good players as you see on YouTube, they are usually the Top people that you only meet at Rank (18?) or higher.
The online ranking system has also changed. In the past, you got points when you won a match and lost points when you lost a match. With enough points you rise or fall in ranking. In Tekken 8, you do not lose any points in the first 9 ranks if you lose, so you are not at risk of demotion. In other words, you can feel free to experiment here on what works and what doesn’t.
If you still get a good beating from someone, you can always look back at the replay. You can watch all your matches and it will be explained what you can do best in a specific situation and you can even practice on them.
And yes, Tekken is indeed a deep fighting game, you can practice/train for 40 hours per character to get the best combos or figure out which move to use in each situation. But for people who don’t want that, I think there is plenty of room to have fun too. Think of it like chess, many people like chess and will probably play a few games. There are few chess masters, you won’t often meet them unless you look them up yourself-