Draining a huge lake is easier said than done though. Even if the bypasses are still up for it they almost certainly were not designed for that. In addition, every bridge and river bank downstream would need to be checked and possibly reinforced for a much higher flow rate.
Btw, the time scales are quite long even if it can be done. Filling Lake Mead took from late 1934 to may 1937 or about 2.5 years. Assuming the downstream infrastructure can take double the normal flows, you'd need 2.5 years to drain and another 2.5 years to refill after the maintenance is done. The opportunity costs are huge, it seems at least worthwhile to check if you can do the works underwater with robots or something like that.
And then there's the inconvenient issue that in many places we've built on previously non-buildable land downstream.
Drain down, dig a bypass, temporary dams inside both sides of the bypass, do the offline / bypassed work, reverse the procedure. Maybe the second time we'll have the foresight to make the bypass more easily serviceable again.