I would much rather them use existing tried tech and incrementally advance them rather than try a radical new design.
Except that they're not. Those solid boosters? They're "based on" Shuttle SRBs, not identical to them. Several segments longer, meaning higher internal pressures, different burn characteristics, etc. If you don't think that's going to take extra years of testing, there are several bridges I'd be happy to sell you.
Ditto for any other technologies that they're basing stuff on rather than reusing identically.
The SLS isn't also known as the "Senate Launch System" for nothing. NASA's role should be to try radical new designs, not serve as a conduit for senators to shovel pork to their constituents.