The general consensus on the Z lenses is that they are optically superior to the F mount lenses. Yes, they use more plastic than the old school lenses. This makes them lighter and potentially less robust than their all metal counterparts. The newer designs and larger Z mount allow the engineers to design for better optical performance. Newer manufacturing techniques also allow for tighter tolerances and more consistent parts.
Will they last as long as the old school lenses? I sincerely doubt it. Between the new materials and the increased use of electronics, they will be unlikely to last the 30 - 40 years or more of use that their older siblings were capable of.
The other issue here is that you are looking at one of the lower cost lenses here. When I compare my daughter's Z DX 12-28 F3.5- 5.6 to my Z 24-70 F4, the 12-28 seems rather light and cheap. Of course, the 24-70 is a DX lens and costs about a third of the 24-70. The 24-70 is an S-line lens and feels robust in the hand, despite having a lot of plastic parts. Not all plastics are created equal.
I started out with a handed down D3400 equipped with an 18-55 kit lens. It is extremely inexpensive, very light and even has a plastic bayonet mount. Considering the price, it is an excellent performer optically and has stood up to a fair amount of use and even some abuse.
Use the lens for a while and give it a fair shot. It is not going to be up to the $2K+ Z 24-70 F2.8 S, but it probably outperforms the 18-55 on my D3400. I believe you will find it probably optically outperforms most of the lenses from the 80s and 90s in most categories except durability.
