I mean Brook Lopez will be a better player than Russell in 2017–18, so I don’t agree with your Nets sentiment. They will almost certainly have a bad enough record that it leads to yet another top 5 pick for the Celtics in next year’s similarly loaded draft. They were the worst team in the NBA by four full games last year. Even if you think Russell is more valuable than Lopez next year (which again, I don’t), is he really worth 4 extra wins?
Second, as others have said, that’s why the Celtics included the 2019 SAC/PHI pick in that trade. Worst case they get a top 10 pick in 2019 (most likely top 5) if they don’t get the 2018 LAL pick. Personally, I think LAL would be stupid to trade for PG now instead of just signing him next year. Why would they give up a pick with the potential of #2 this year? It’s not like they’d contend for a title even with PG. Assuming they don’t get PG until next summer, it is extremely likely that the Celtics get that pick between 2 and 5 next year. If anything, the Lakers would have to hope they get the #1 pick next year as opposed to being good enough to pick 6th or later.