Forever speakers around $5000

I agree that Spendors may check all your boxes but I’d go with the A7s over the D7s if you found the KEF LS50s "shrill."

BTW, the LS50 is a balanced speaker if listened to as KEF intended, off-axis and below ear height. Unfortunately, many dealers lack this insight it seems.

You should also look at the Elac Vela range, which are alleged to have an airy yet non-fatiguing top end.

Another speaker I like is the Martin Logan Motion 60XT. They’re much better than MLs entry-level ESLs, but might not be as dynamic as you're wanting. A side warning regarding AMT tweeters: just like soft or metal domes, there is a wide variance in quality among manufacturers. ML makes some nice AMT tweeters, while some AMTs sound like cheap boom-box tin.

I very much enjoy my pair of Maggie .7s  but the Spendor A7s will better them in most regards and their price reflects that - bigger soundstage, greater detail, deeper bass, sweeter midrange... Admittedly, that's an assumption I'm basing on my demos of the smaller A4s which are the most well-rounded, can't-go-wrong speakers I've ever heard. 

I have Spendor 2/3s paired with stereo subs and the soundstage is only limited by the width of the room. 

NO listening fatigue, period. THAT is one of the important tests for "forever" speakers.

Try that with ANY OTHER box speaker, electrostatics, etc. Unfortunately, you will want to stop listening to any other brand eventually. Never true with Maggies unless your source material is unlistenable. NO speaker fixes that!

Also, they reproduce what you send them. If you can live with that, your quest is over.

As an owner of Maggies I must say this is a very misleading post.

It’s true that Maggies require good gear to get the best from them but they have their limitations and are far from perfect. Many speakers previously mentioned are more resolving than any of the <$5K Maggies, regardless of sources and amps. A good example is the Spendor A4 and likely even the OP’s LSAs. The Magnepans certainly have their strengths but ultra high levels of detail and pinpoint imaging are a couple areas where they fall short compared to many in the $5K segment.

And of the three Maggie models I’ve experienced (1.7i, 3.7i and .7), for whatever odd reason, only the .7s offer a balanced presentation with real bass slam. The 3.7i do indeed produce fatiguing highs, while the 1.7is must be cranked over 80db to get any real bass dynamics, and lack resolution.

Where all Maggies seem to excel is disappearing as sources of sound and transient speed. For those reasons I love my .7s but like any speaker, they’re not an end-all, be-all. Some of my <$5K box speakers can throw a much larger soundstage and showcase greater detail.

while you’re likely chasing the endless rainbow like we all are...
how about one of the traditional larger boxes from the likes of Harbeth, Stirling, or Spendor ?

These brands tend to be final destinations for many listeners, more often than the typical stiff-cabinet, narrow-baffle floorstander. They're not the last word in any one performance metric but their combinations of talents are hard to beat. I think they're best appreciated by listeners who can appreciate realistic tone/timbre. 

Many assume the flexible cabinets are designed to add an artificial warmth but it's really intended to lower the resonant frequencies where they're far less audible. In contrast, many stiff-cabinet floorstanders have resonances >500Hz which makes them too easy for the ears to locate. Thus, they don't "disappear."  My Spendors and Stirlings are about as stealthy and transparent in this regard as my Maggie .7s. 

I beg to differ on Maggie's 3.7i not having pinpoint imaging and detail as mine are extremely detailed and image extremely well. Each instrument is crystal clear with exact placement in the mix. If you don't hear this I suggest you check your speaker placement of source material and other associated equipment.

Regarding the 3.7is, yes, they're plenty detailed thanks to the pure ribbon tweeter but the ones I heard were overbearing and bright in the top end, not balanced like the 1.7s and .7s. The latter two certainly do lack some last bits of air and detail, and when a multitude of box speakers surpass them in those areas, in multiple rooms, among multiple combinations of ancillaries, logic would dictate it's not the upstream gear. 

None of the Maggies I've heard do pinpoint imaging like a good coaxial dynamic driver. However, I really enjoy them for their microdynamics where most pale in comparison.

Anyway, back on topic.
OP, if your space allows, you 
should give Spatial Audio speakers a try. Clayton is a pleasure to deal with and his speakers really do compete with models 2X their price.