Define A "Good Topical Collection"

I swing both ways. I've removed the stamps that don't fit the topic from my Esperanto collection, but I keep full sets in my Runes and Runestones collection and my Tolkien collection.

In the case of the Runes and Runestones collection, there is one Icelandic set where I've only bought the three stamps with runes on them (Scott #152, 157, and 162), because I don't want to spend $30–50 each on stamps that aren't in my topic just because they're part of a set. Price is also why Iceland #162 is MLH rather than MNH.

A complete set is one that has all stamps in it as defined
by a stamp catalog. Didn't see a direct answer to that question.

It can be tricky. What's a set of Machins, for example? More detailed catalogues will break down issues into groups that appeared in particular timeframes. A general one might list them all in one long "set".

A set of Machins is however your favorite catalog lists them. I doubt any major catalog lists a long set of Machins, given they were issued over a period of 50+ years. It's like saying "what is a complete set of Sudan Camel Post stamps?" This set was issued unchanged, like the Machins, for about 50 years. Yet, most catalogs break the Camel Post stamps into sets based on years of issue. Scott's does break up the Machins into various sets, though I fully understand that Scott is neither authoritative nor complete for the Machin issues. But you could collect them by Scott's and find plenty of US sellers and buyers who would understand what you are communicating through Scott numbers.

Given that this thread is about thematic collecting, I offered a simple answer since most material collected by theme is not complex or long like Machins or other definitives. Sets that contain thematic material, even if definitives, are still defined as sets by the various catalogs.

Unless your theme is 'Royals' or 'Permanent Stamps' Machins are not much of a thematic subject.
The Spanish 'tourism set' from the 1960s and 1970s is an example.

Correos issued stamps themed 'Paisajes y Monumentos.' In 1964 and 1965, Correos issued 10 single stamps in this series. Edifil lists them as two sets: one for each year. In 1966, it issued two sets of five stamps. It started numbering the stamps and continued until 1970. After a two-year break, it started to issue shorter sets again. The name changed.

If you collect 'Monuments,' 'Churches,' or 'Landscapes' neither the way they were issued, nor the way they have been catalogued defines a 'complete' set. Yet, the complete set of any will include stamps that are part of most, if not all catalogued sets. Then again, some of the 'Churches' or 'Monuments' or 'Landscapes' were issued separately but catalogued within a longer set.

Why should a catalogue editor or dealer determine what is complete?

If you only want a 1 pta stamp when the 'set' costs 10 ptas, why throw away 9 ptas? Today, these sets are 'cheap.' At the time, 9 ptas. was a lot of money spent on what you do not need. Any new set added will reflect today's issuing policy. Any stamp you do not need will be expensive today and almost certain to be cheap in the future.

Of course there are many instances where this happens. Michel for example lists stamps as they are issued so obvious sets get broken up if issued over time. Scott had a policy change, though I can't recall what year it occurred, where they no longer set aside numbers for additional stamps in long sets. Instead, they started to list additional issues for a set by the year these additions were issued.

That said, Scott usually tells you to "See 1107-25, 1248-62, 1418, and 1536." Michel does the same, at least in my experience. So while the set may have been issued in pieces, catalogs often tell the user which other stamps are part of a set. Likewise, some collectors may find themselves in a position where the have certain parts of a set, and not other parts. They won't wish to buy everything together and create duplication, they will want only the parts they are missing. So the catalog listings of a "set," the ones where only 1 or a few stamps were issued, provides a common language between buyers and sellers on trading for only the stamps a collector wants and is missing from a broader, larger set.

Agree with NSK about most very early definitives and modern ones like Machins. Unless you collect monarchs or other royals, there isn't much to be had for topical collectors. But even here, there are likely QE2 thematic collectors, and GB is not the only country to have issued Machins. Hong Kong, for example, has issued quite a number of definitives that depict the Machin bust. But even in some earlier issues, e.g. 19th century Liberia or early 20th century Somali Coast and Belgian Congo, there are some definitive stamps that were pictorials and thus sought after by some topical collectors. Pictorial definitives have been common since the 1930s.