Some things, in the hidden layers, need to evolve/modernize to ensure it continues to be compatible/workable with the environment evolving around UbuntuMATE. I am sure that the only reason Yaru was implemented was because the way HTML/XML/DOM was evolving was forcing the need to implement a GUI layer "engine" concept which ensured that the "headland" packages, upon which the UM DM was dependant, did not leave our much-love GUI broken, unless something changed. 
Some things just can't be avoided, as the saying goes, like taxes or death! I think Yaru was one of those inevitables to ensure an interraction as close as possible to what we had before. Imagine if @Wimpy had not made Yaru happen, keeping UM's need predominant in his mind but, instead, the time came when we were forced to adopt a similar layer but conceived by someone, for example, whose vested interest was more in line with Elementary OS !!! While I did not "enjoy" the forced switch, I came to accept that it wasn't forcing a complete rethink at the GUI level (only at the underlying ease of customization).