Several letters written to Vasari and the Grand Duke in this and the next two years show that his heart was set on finishing S.Peter's, however much he wished to please his friends and longed to end his days in peace at home.

"I was set to work upon S.Peter's against my will, and I have served now eight years gratis, and with the utmost injury and discomfort to myself.

Now that the fabric has been pushed forward and there is money to spend, and I am just upon the point of vaulting in the cupola, my departure from Rome would be the ruin of the edifice, and for me a great disgrace throughout all Christendom, and to my soul a grievous sin.
Pray ask his lordship to give me leave of absence till S.Peter's has reached a point at which it cannot be altered in its main features.
Should I leave Rome earlier, I should be the cause of a great ruin, a great disgrace, and a great sin." To the Duke he writes in 1557 that his special reasons for not wishing to abandon S.Peter's were, first, that the work would fall into the hands of thieves and rogues; secondly, that it might probably be suspended altogether; thirdly, that he owned property in Rome to the amount of several thousand crowns, which, if he left without permission, would be lost; fourthly, that he was suffering from several ailments.

He also observed that the work had just reached its most critical stage (i.e., the erection of the cupola), and that to desert it at the present moment would be a great disgrace.
The vaulting of the cupola had now indeed become the main preoccupation of Michelangelo's life.

Early in 1557 a serious illness threatened his health, and several friends, including the Cardinal of Carpi, Donato Giannotti, Tommaso Cavalieri, Francesco Bandini, and Lottino, persuaded him that he ought to construct a large model, so that the execution of this most important feature of the edifice might not be impeded in the event of his death.