Kind of agree with Northwind but would err more towards the opinion that if you can't ride something on a hardtail then a full sus won't magically make it rideable.
You can often ride faster on a full sus and save some energy on the longer rides but in terms of actually riding a line I wouldn't say there is much in it personally. A dropper post may be a practical help, tapered steerers/ different wheel sizes/ etc make naff all difference to a line you might or might not try and ride.
The biggest issue is mental and the commitment required, especially on the steeper stuff you mention... maybe a full sus would give you that confidence even if in reality there is little difference (unless you are doing a rather large drop in to a very rough landing or similar)?
I also personally like short travel full sus or hardtails to feel a bit more "connected" to what is going on rather than let a long travel bike dull a lot of what is happening under the wheels (a generalisation I realise!). That is just personal and I am sure a longer travel bike might be faster on some descents but if don't like the look of a line then the amount of travel the bike has won't change that
There really is no compensation for skills and technique but perhaps the mental benefits of confidence and having a shiny new bike are worth it As others have said, a lot of it comes down to experience too- if you ride a certain line at a certain speed once you kind of get a better idea of what is possible and the confidence grows from there