No assassination attempt on Lenin. He lives longer. Out maneuvers and eliminates the Stalinists before handing the leadership to Trotsky.
Even assuming the assassination attempt shortened Lenin's life (which is unclear) there are a number of problems here, not the least of which is that Lenin never showed any desire to "hand the leadership to Trotsky." "Remove Stalin as General Secretary" is not the same thing as "give his power to Trotsky." The "Testament" contains criticisms of Trotsky as well as Stalin--indeed, of all the Bolshevik leaders (except Lenin himself of course). "He [Trotsky] is personally perhaps the most capable man in the present C.C., but he has displayed excessive self-assurance and shown excessive preoccupation with the purely administrative side of the work."
https://www.marxists.org/archive/lenin/works/1922/dec/testamnt/congress.htm
This may not seem like very severe criticism, but it alludes to two of Trotsky's least popular characteristics: his arrogance, and his inclination to rule by fiat ("administration" being a euphemism for this).
Lenin also alludes to Trotsky's "struggles against the C.C. on the question of the People's Commissariat for Communications" and in one sentence scores hits against Zinoviev, Kamenev,
and Trotsky:
"I shall just recall that the October episode with Zinoviev and Kamenev was, of course, no accident, but neither can the blame for it be laid upon them personally, any more than non-Bolshevism can upon Trotsky." (In other words, Lenin was pointedly reminding his comrades of the things about Zinoviev, Kamenev, and Trotsky that in the next breath he says shouldn't be held against them...)
The other Bolsheviks don't fare much better, e.g.: "Bukharin is not only a most valuable and major theorist of the Party; he is also rightly considered the favorite of the whole Party, but his theoretical views can be classified as fully Marxist only with the great reserve, for there is something scholastic about him (he has never made a study of dialectics, and, I think, never fully appreciated it)." Translation: We all love Comrade Bukharin, but let's face it, he is no politician.
If there is anything to be drawn from this, it is that Lenin did not consider
any of the Bolshevik leaders to be worthy of succeeding him. Some of the others seem to be guilty of things at least as bad as Stalin's "rudeness."