the specter of the Highway 40 rebuild
New I-64 won't even begin to move any dirt until 2007. With Cross-County MetroLink scheduled to open in Fall 2006, I don't think rebuilding 40 qualifies as a factor to consider in Metro's delayed project. You may have a point though, in that once 40 is partially shut down for reconstruction, folks (in mid-county especially) may be a little thankful that the parkway and extension offer some choice.
with rising gas prices
If you think gas prices will be significantly falling after Cross-County opens, then I commend you for such hopeful optimism. If anything, I think $2-plus prices are here to stay, if not increase even more.
Overtime, Metro's credibility will slowly be rebuilt. Once the new extension opens and people realize how many more places are then connected to MetroLink, I think another sales tax has a very good chance of passing.
However, it will be the debate over which lines to build next that may spoil another Prop M, not Metro's Cross-County management. The City may even vote down a proposal (though the last one barely passed and may not require a new election but didn't go in effect due to the County's failing).
For example, if the next line appears to the public as Clayton to Westport, then even pro-transit City voters may decide not to support a tax they see as paying only for County expansion. Vice-versa, County voters may vote against a Northside/Southside extension. Or even in a compromised Northside/Westport line, southside voters could spoil the City vote.
Madison County recently experienced the same dilemma, concerned if the line would serve the Riverbend communities or greater Edwardsville area. Though MetroLink is popular, it's biggest flaw for greater public support is how it has been built incrementally to only sub-sections of our region.