Delphi was a decent product, but it was always oversold.
Borland's strategy was terrible from day one:
They allowed their C++ Compiler to stagnate for the better part of 2 decades. This basically ceded a large part of the market to Microsoft.
They switched to Enterprise pricing, which eliminated their biggest advantage - their pricing. The low prices are the only reason why Turbo Pascal ever became a thing...
Elimination of the Standard Editions. The Starter is basically a super-expensive trial version.
Moving the C++ toolchain out of the traditional (similar to Visual C++) IDE onto the Delphi IDE was a bad move, as that IDE was pretty terrible for C++ development (they didn't have another a decent C++ IDE until BDS 2006, which was buggy as hell).
Moving Borland C++ to the Delphi IDE and making it yet another VCL development environment basically caused their products to cannibalize each other. Most Delphi Purchasers would not buy C++Builder and vice versa, since the products were otherwise completely interchangeable. This wasn't the case with tools like Visual C++ and Visual Basic, so Microsoft completely avoided that - which is why both Visual Basic and Visual C++ enjoyed massive popularity while both Delphi and C++ plateau'd and eventually started falling off.
There were other issues, like the Borland Database Engine (BDE) and the deployment issues it can cause when you have multiple applications getting installed on the same machine that use it. I have to roll my eyes every time people pretend as if Delphi/BCB App deployment was some sort of utopia compared to a VB or VC++ MFC application.
If your application valued performance, then Borland's tooling lost by default because Microsoft Visual C++ produced vastly better performing code than both Delphi and C++Builder - and still does.
They got lazy, and even the Delphi language started to fall behind. They didn't get things like Generics until 2009, and it took them forever to move to Unicode. How long did it take for them to develop[ 64-Bit toolchains? Microsoft also had cross compilers for ARM, Itanium, and other platforms for over a decade before these tools implemented anything similar.
I actually thought Kylix was a good idea. The pricing, however, was hilarious. As were the some of the gotchas in the licensing due to their use of Qt.