"The new Severn Bridge is not as long as the old one".

replace 'long' with any other "adjective" such as 'big', 'hot','ugly'.
as "adjective" as - is a comparison
so "adjective" as - does not make sense . ( So big , so tall...so is synonymous to very)
If you explicitly use the phrase 'compared to', then it makes sense.
"The new Severn Bridge is not so long,compared to the old one"
OR
"The new Severn Bridge is not so long" ( compared to nothing or in general or based off the context of a previous sentence , eg: "The old bridge is long".)

"The new Severn Bridge is not required as long as the old one does not fall"
Replace the "idiom" 'as long as' with 'so long as'
"The new Severn Bridge is not required so long as the old one does not fall"