Mughal dinasty is probably the richest Indian dinasty and most probably the richest dinasty ever comparable to Byzantine empires. The older Indian dinasties before Moghul are not that rich or successful otherwise tourists will be visiting their buildings like pyramids in Egypt or Aya Sophia in Turkey.
It is not surprising that Mughal Empire was rich for the ruling class. It imposed the Jizya tax on non-muslims. https://en.wikipedia.org/wiki/Jizya
The jizya is tax for non Muslim and for Muslim they have got zakat and other normal taxes. It is not like Indian do not have any tax nowadays.
The Delhi Sultanate (12th-15 century) was recognised for its oppression of Hindus. Muslim texts of that period even encourage oppression. https://en.wikipedia.org/wiki/Persecution_of_Hindus#Delhi_Su...
The Muslim texts of the Delhi Sultanate era treated Hindus with disdain, remarking "Hindus are never interesting in themselves, but only as converts, as capitation tax payers, or as corpses".
"The Muslim commanders of Delhi Sultanate regularly raided Hindu kingdoms for plunder, mulct their treasuries and looted the Hindu temples therein"
It was certainly one of the dark ages, unless you happened to be an Islamic conqueror! Comparing that era against modern India is hopefully a joke ? Perhaps you only meant to compare Muslim Nobles in that era against modern Indian citizens ? (Even then the life expectancy was lower). Being a Hindu citizen in that era meant you (and your family) could be enslaved for any reason and your village/town slaughtered to the last man, your template desecrated and looted. There are thousands of such documented cases.
Oh, btw, the Zakat tax in that era was negligible compared to Jizya. Can dig up several sources on this too, if needed. If you didn't pay Jizya, you were killed or enslaved. You were freed from Jizya only if you converted to Islam.
Dark ages meant that there is a sharp decline in the quality of life especially in the knowledge and scholarly departments.
If the Sultanates are really bad as you have described there should be no Hindu left in the India sub-continent but the last time I checked they are still the majority. The Sultanates can easily pull a Spanish Inquisition style by killing or forced conversion. They can drive away the Hindu away like what the Spanish Christians did during inquisition to the Muslim and Jews by not spare anyone of them and eliminated all the mosques and sinagogues from Spain[2].
It fits all metrics of a dark age. A distinct minority of the population being prosperous does not repudiate this.
The slaughter of Indians in that era even exceeds the Spanish Inquisition. There were regular massacres of over >100K folks after a city surrendered. (Ex: Ghiyas ud din Balban of Delih Sultanate).
The population of Delhi under the Delhi Sultanate was utterly exterminated by Timur. It took another century for the population to recover. He killed 17 million people in his campaign.
https://www.nationalheraldindia.com/time-prism/december-17-i...
https://en.wikipedia.org/wiki/Timurid_conquests_and_invasion...
A full post on the large-scale massacres during that period are just too many to list here.
Believe it or not, Timur probably killed more muslim in his entire campaign than non Muslim.
The Delhi Sultanates created the Hindustan languages and started the Indo-Islamic architecture that reached its zenith during the Mughal Empire[1]. The Delhi Sultanate is also noted for its integration of the Indian subcontinent into a global cosmopolitan culture[2]. The Sultanate is a few forces in the world that has successfully repelled the Mongul catastrophic invasion. FYI, Mongul successfully decimated Baghdad of Abbasid empire and burnt it to the ground including the largest library known to the world at the time.
[1]https://en.m.wikipedia.org/wiki/Delhi_Sultanate
[2]Asher, C. B.; Talbot, C (1 January 2008), India Before Europe (1st ed.), Cambridge University Press, pp. 50–52, ISBN 978-0-521-51750-8