The paper was a good read. I was kind of dreading it'd be another tired rehash of attacking straw men from someone who thinks having used Java or C++ makes them expert on OOP.
In the early history of smalltalk Alan Kay talks about how he was never entirely satisfied with inheritance, and having inheritance in the language (quoting in full, hard to link to):
A word about inheritance. Simula-I had neither classes as objects nor inheritance. Simula-67 added the latter as a generalization to the ALGOL-60 structure. This was a great idea. But it did have some drawbacks: minor ones like name clashes in multiple threaded lists (no one uses threaded lists anymore), and major ones like rigidity in the extended type structures, need to qualify types, only a single path of inheritance, and difficulty in adapting to an interactive development system with incremental compiling and other needs for instant changes. Then there were a host of problems that were really outside the scope of Simula's goals: having to do with various kinds of modeling and inferencing that were of interest in the world of artificial intelligence. For example, not all useful questions could be answered by following a static chain. Some of them required a kind of "inheritance" or "inferencing" through dynamically bound "parts" (i.e. instance variables). Multiple inheritance also looked important but the corresponding possible clashes between methods of the same name in different superclasses looked difficult to handle, and so forth.
On the other hand, since things can be done with a dynamic language that are difficult with a statically compiled one, I just decided to leave inheritance out as a feature in Smalltalk-72, knowing that we could simulate it back using Smalltalk's LISPlike flexibility. The biggest contributor to these AI ideas was Larry Tesler who used what is now called "slot inheritance" extensively in his various versions of early desktop publishing systems. Nowadays, this would be called a "delegation-style" inheritance scheme [Liberman 84]. Danny Bobrow and Terry Winograd during this period were designing a "frame-based" AI language called KRL which was "object-oriented" and I believe was influenced by early Smalltalk. It had a kind of multiple inheritance—called perspectives—which permitted an object to play multiple roles in a very clean way. Many of these ideas a few years later went into PIE, an interesting extension of Smalltalk to networks and higher level descriptions by Ira Goldstein and Bobrow [Goldstein & Bobrow 1980].
By the time Smalltalk-76 came along, Dan Ingalls had come up with a scheme that was Simula-like in its semantics but could be incrementally changed on the fly to be in accord with our goals of close interaction. I was not completely thrilled with it because it seemed that we needed a better theory about inheritance entirely (and still do). For example, inheritance and instancing (which is a kind of inheritance) muddles both pragmatics (such as factoring code to save space) and semantics (used for way too many tasks such as: specialization, generalization, speciation, etc.) Alan Borning employed a multiple inheritance scheme in Thinglab [Borning 1977] which was implemented in Smalltalk-76. But no comprehensive and clean multiple inheritance scheme appeared that was compelling enough to surmount Dan's original Simula-like design.