Method that will work. The way it is used above will not work.
declare @str varchar(50)='79136'
select
case
when @str LIKE replicate('[0-9]',LEN(@str)) then 1
else 0
end
declare @str2 varchar(50)='79D136'
select
case
when @str2 LIKE replicate('[0-9]',LEN(@str)) then 1
else 0
end