Argentine default: "Rights upon future offers"
This is a good question! It turns out that exactly as you predict, if you get the transcripts or recordings from the meetings with judge Griesa in NY, some of the representatives of the non-holdouts said they were willing to sign off on the RUFO clause, voiding it. The logic, as you describe is that this would free Argentina to pay the holdouts, letting Argentina continue its payments to the non-holdouts too, without a risk of the debt ballooning suddenly. This would have saved everybody lots of headaches!

Argentina's government at the time did not pursue this option, perhaps because they saw more political gain in being the victims of 'the US empire's courts' than in solving the problem. The lack of professionalism and interest in solving the problem that their executives displayed during this episode suggests this is the main reason.

Alternatively, it's plausible that they did not pursue this option because they thought that default was so terrible to non-holdout investors, that these would be of help in pressuring the holdouts to accept a lower payment from Argentina in negotiations to avoid default.

In any case, neither strategy worked. The government lost popularity and was kicked out of office, and the new government eventually reached an agreement beneficial to the holdouts.

One, more thing: the Rufo clause actually wasn't such a big deal because it was set to expire soon! So, the non-payment appears to have been a big circus for the Argentinian president then to try to score some points with the more manipulable fraction of that country's population.