Maybe you have been living under a rock but lithium is cheaper than sealed lead acid nowadays, especially in small sizes. And unless you go for an even more expensive premium brand, lead acid nowadays generally doesn't last past 3-5 years even when kept under optimum conditions. Contrast that to (name brand) 18650s which easily last past 10 years when treated well.
Size and non specified brand specific

Stop making stuff up to suit YOUR narrative. Lithiums are across the board offer less capacity for more $.
I have AGM's doing well after your apparent time frame. I still have a few NiCads that are well over 20 years old that doesn't make them a good option because of this. Some of my Lithiums are close to or might be over 10 years old in spite of regular high current use and frequent charged state storage. I have also had LiPo's in particular (good branded ones) puff well under max current after a few uses
Don't throw insults you can't back with actual fact!
The diode is for discharge. The charging stops by means of the supply voltage being limited. I suppose one could use a current limited LDO instead of a resistor if a fancier solution is desired.
So you are now suggesting a voltage fully regulated power supply and LDO's and ..... instead of the $3 Balancing Charger board (good for 500mA-1A)? Stop trying to weasel this out as a good solution it is NOT. Use a proper charger that exists and works instead of an imagined make believe bad solution.