"The architecture of pre 20-th century was as flashy and kitschy as the people of the day could make it with the technology at their disposal."That's somewhat of an overstatement. I understand why people like modernist architecture but that's a different matter altogether to plastering over any architecture—modern or ancient—with a mishmash of billboards and ad posters. Modern architects don't want their works defaced either.
On the matter of façades and ornamentation of pre 20th Century architecture, no doubt some were ostentatious by the standards of the day, as are many buildings of the modern era, but many were not. Moreover, what passed for 'ostentatiousness' in one era would be matter-of-fact in another. For example, if you took late Victorian architecture back a hundred years say to Georgian era (ca ≈1720—1820), it'd be considered scandalous.
That said, there were and still are longstanding rules about ornamentation on buildings such as friezes and moldings. These conventions have stood the test of time and go right back to Ancient Greece and Rome. For instance, curves such ovolo, ogee and cyma just to name a few haven't changed one iota since those days several millennia ago (I have woodworking tools that cut those shapes precisely right to the correct mathematical curves—that's how fussy many people still are even these days—much more so than I am).
https://en.m.wikipedia.org/wiki/Molding_(decorative)
https://en.m.wikipedia.org/wiki/Frieze
They've stood the test of time because people of every age—ancient and modern—consider them beautiful and harmonious.
Of course, that doesn't mean modern stuff is bad, the objection is that ads screw up both ancient and modern architecture because they don't pay credence to the existing architecture, they are there solely to distract for the purpose of selling, that is that they are purposely disruptive and break the harmony of the landscape whether it be modern or ancient.