It is very hard to imagine any legalistic self-incumbency of the rulers to resurrect fallen places of worship - for the subjects. Most versions of the covenant of Umer (ra) with the Dhimmis, which serves as an important formative document, are devoid of any bindings on the caliphate to help repair any churches. In fact, some versions bar the Christians from building new churches, some versions even go as far as to bar them from repairing dilapidated buildings. The important point is that there exists no single version of the covenant and variations range from very light-weight terms to very wearisome as detailed in Shafi's Kitab-ul-Umm. For details see Caliphs and their Non-Muslim Subjects: A Critical Study of the Covenant of 'Umar.
An interesting observation is that the lack of stipulations specific to helping the Dhimmis practice their religion can go both ways. A glance through the history reveals precedents along both lines. I'm quoting from Caliphs and their Non-Muslim Subjects: A Critical Study of the Covenant of 'Umar.
'Amr b. 'As gave to the Makaukas part of the Lake of Habash as a
burial ground for Christians. In 60 or 61 part of the great church in
Edessa was thrown down by an earthquake. Mu'awia ordered it to be
rebuilt.
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The first church in Fustat was built in Harat ur Rum while Maslama b.
Mukhallad was governor, between 47 and 68. When 'Abd ul 'Aziz founded
Hulwan he allowed two Melkite servants of his to build a church there,
and the patriarch built one also, because he had to pay his respects
to the governor there. 'Abd ul 'Aziz told some bishops to build two
convents there, and he allowed Athanasius, his secretary, to build a
church in Kasr ush Shama'.
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Some chamberlains of Mamun restored the church of the Virgin at al
Kantara, and two servants (farrasti) obtained permission to build one
on Mount Mukattam, because those in the castle were too far away. In
this reign Bukam, a wealthy Christian of Bura, built many beautiful
churches in his native town.
These precedents however appear to be exceptions rather than normalcy, as the amount of destruction appears to outweighs the amount of construction. Legally, all places of worship were to be protected and the destruction or forced acquisition prohibited as long as the covenant was held. Muslims destroying churches was as much an exception as building new ones, from the covenant of Umer (ra) - if and only if the covenant actually stipulated a ban on building new churches.
Its been centuries since the term Dhimmi has ceased to exist except in theory, and therefore the contemporary legal standpoint is expected to be the same as that of the middle ages. Contemporary issues surround the very justification of Dhimmi status, terms of contract are only consequential details.
Disclaimer: Based on the material from the book referenced.
