This sounds like a great way to ensure the high levels get even less support.
It might have, if any edition had ever offered significant support for high level play- adventures, especially.
The proposal to divide the Players Handbook between Lower Tiers and Upper Tiers has several advantages.
Having a Players Handbook that only attends to levels 1 to 12 appeals to players who find the upper tiers less manageable or less grounded. This low-level appeal is legitimate as a gaming-style preference.
There are also practical considerations. Statistically, most 5e games end roughly around level 8. So a separate book that ensures that these levels are solid, thematically and mechanically, is financially sound. Removing the space for levels 13 up (and spell slots 7 and up) allows more room to focus on levels 1 to 12, and to get its flavors and mechanics diverse and robust.
The low tiers include levels 9 to 12, as a four-level Master tier. Even tho games now conclude by 8, the Master tier is an inviting, limited, manageable, temptation to press on a bit longer. This is when player characters leave their mark in the world, so future characters can benefit narratively (and perhaps mechanically) from the accomplishments of previous characters. The Master tier enriches the D&D experience and encourages immersion in the fictive world.
The levels of the Master tier correlate with the old school "name levels" of AD&D, including the building of institutions and the formation of communities. Where the old school gaming engine failed from levels 13 and up (and started going awry 9 to 12), the characters beyond 12 were relatively rare. Together, the Student, Professional, and Master tiers − levels 1 to 12 − also recall the nostalgia for most peoples experiences of old school D&D.
A Low Tier Players Handbook, for levels 1 to 12, will be a solid product and will sell well.
At the same time, I like high level characters. I want a dedicated Upper Tier Players Handbook that will make levels 13 into epic more enjoyable.
The situation right now is, the upper tier is an afterthought. Possibly it is fair to say there is zero support for upper tier characters. The high levels are little more than cut-and-paste from previous editions, while deleting features from them that were truly disruptive to gameplay. There seems little or no active support. For example, for spellcasters who specialize in Plant themes, there are no Plant spells at slots 7 and higher. Similarly for other spell themes. Many high level spells are terrible. A separate book forces designers to consider if a spell really is worthwhile alongside the other spells in the same slot. Relatedly, Fighter players feel their characters are inadequate at these levels. So, while 5e has some kind of echo from previous editions, there is little or no actual support.
Right now, if one simply transfers upper tier content from the current Players Handbook to a separate book, that separate book would be unpublishable. It is anemic at best with glaring omissions and doubtful game engine balance. Moreover, the "small town" flavor of the lower tiers makes less sense in the upper tiers. The upper tiers are about nations, populations, planets, and planes.
(Upper tier is cosmic. It is probably worthwhile to write "level 25" on ones character sheet, in the sense of having completed level 24. But what does level 25 mean? At this point, the powers of such a character are little different from being a DM.)
Nowhere does the current Players Handbook instruct the players and the DM how to navigate the adventures at this scale of magnitude.
If the Upper Tier Players Handbook exists at all, it forces the 5e designers to support these levels, both conceptually, mechanically, and with appropriate options at each level.
The UT Players Handbook will sell well enough. It will be core, and at the least the completionists will buy it. But if it turns out to be fun, others will buy it too.
