They had to start a new character at level 1 anything else would've made no sense at all and you clearly don't know what you are talking about if you think they could've continued the Bhaalspawn story (at level 31/40+) after Throne of Bhaal.
But it's funny because you basically sound like me before I actually gave it a shot, I didn't think they could've possibly pulled this off, but they did, as unlikely as that sounds in the gaming world today. In many aspects it is even closer to the table top experience, which to emulate, was the entire point of the original, but some things couldn't be done because of technical limitations at the time.
Just give it a fair shot, it's worth it.