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Look at it this way. Traditional, european-style subways offer little more in capacity and speed than a well-built modern LRT. And with building transit infrastructure being so expensive nowadays for some reason, it figures that it doesn't make sense to build heavy rail for those kinds of demands. If you are going to build a subway it should be built to a massive, asian style scale, with 8+ car trains and stations at least 1km apart, and covering much farther distances than just within a city, reaching out into the suburbs. Basically a somewhat smaller scaled commuter rail.
LRT is good for the fine distribution, as in you change from heavy rail (commuter rail or subway) to local transit (bus or LRT/streetcar). LRT has good capacity and good speed for this, so whenever you need something more than a bus an LRT will do fine. Subways are good as main trunk lines, but not for full coverage, unless the city is really ridiculously large and dense (like Tokyo, London, Paris, etc.). An example of subways intending full coverage and not working out well is Barcelona. You have a subway that's impractical for longer distances because it's relatively slow (especially if you have changeovers), while commuter rail is insufficiently built up to reach S-Bahn type service within the closer suburbs and inner city (more stations and frequency but less than subways). The expansion of this subway is extremely expensive, so it constantly lags behind the actual needs of the city, and the inflexible line pattern often requires several changeovers even for shorter trips. LRT would be much faster to expand, and you could have a variety of services minimizing changeovers, plus changeovers would be at-grade.
