Because against the PLA, the B-1 is slightly more survivable than B-52.
For the PLA against the US, a supersonic bomber will not be much more survivable than a subsonic bomber.
Because of the respective missions they were tasked with during the US war on terror where the B-1 was given more of a CAS function than the B-52 which retained its nuclear deterrent role.
I think you are vastly underestimating the cost, complexity and time needed for such a venture.
If they could somehow get such a project done and finished within 2-3 years, sure it might be worth the money.
But given the complexity of those airframes and given the scale of the modifications you describe, it will probably be closer to 10 years.
Yeah I know questionable source but there's some good info there.
1. In 2011 B-52s got modernized electronics and Link 16. It cost $12 billion USD. It was done in time for the next upgrade.
2. In 2019 B-52s had a new radar contract. They had their old radar replaced with an AESA. Contracts awarded in 2019, project starts delivery 2023.
3. Now they're starting a weapons integration program. That will take 10 years.
China's MIC typically moves faster than US. Russian MIC moves slowly due to lack of funding. With Chinese funding, it'll go faster.
Program started in 2014, already delivering.