What's interesting and rightly mentioned in the article is the law was changed in 2004 to also protect the Dem's Senate seat. The Mass. legislature assumed Kerry would be the next President and they did not want Republican Governor Mitt Romney selecting the next Senator.
The succession law was changed in 2004, when Sen. John Kerry, D-Mass., became his party's presidential nominee and Republican Mitt Romney was the state's governor. Before the change, the governor would have appointed a replacement to serve until the next general election.
That would have created the opportunity for Romney to install a fellow Republican in office, a move that Democrats who control the state legislature sought to prevent.
And now when that change does quite work in the Dems favor they of course will change the rules, again. BTW, Ted Kennedy urged the change back in 2004...